Partner's Bid Is Information, Not Code
Euchre etiquette and most table rules forbid signaling. You cannot tap your nose to indicate a void or wink at partner to suggest trump. What you can do is read what partner's legal actions reveal. A pass tells you something. A call tells you more. A pickup tells you the most.
The information value of partner's bid is highest in the first round, when the entire table is committing or not committing to a specific suit (the up-card's suit). It drops in the second round, where the named trump can be any of three suits.
Partner Passes in the First Round
When partner passes on the first round, they are saying their hand cannot make three tricks alone, given the up-card's suit as trump, plus their expected partner contribution.
The pass tells you partner does not hold both bowers in the up-card's suit, and almost certainly does not hold the right bower with two trump (because that is a near-automatic call in most seats). Partner could have moderate strength in the up-card's suit (one trump, even a bower, plus weak side cards), but the hand was not strong enough to call.
The pass also tells you something subtler: partner's strength, if they have any, is more likely in a different color. Players tend to bid the color where their bowers cluster. A first-round pass when the up-card is red is mildly suggestive that partner has strength in black.
Partner Orders Up
When partner orders up the dealer, they are calling trump with you as partner. They expect three tricks between you, but they also expect to contribute most of those tricks themselves (since you have not shown any strength yet).
The order-up tells you partner has a hand consistent with the three-trick benchmark in the up-card's suit. That usually means two or more trump including a bower, or a bower plus a side ace plus a second trump.
Your job as partner is to support. Hold any trump you have for following partner's trump leads, lead any side ace early to get it through, and avoid wasting your high cards on tricks partner is already winning. If you have a strong side ace, lead it first when you take the lead; if you have nothing strong, lead low.
Partner Picks Up as Dealer
When partner is the dealer and picks up the up-card on their own (without being ordered), they are showing a hand that is good in the up-card's suit plus the strength to commit alone. This is one of the most informative bids in euchre.
Picking up tells you partner has at least one trump strong enough to justify the pickup, plus the up-card itself, plus enough side strength to expect three tricks. Often the pickup is on a hand with one bower plus the up-card adding a second high trump, or two existing trump plus the up-card adding a third.
As partner of a dealer pickup, you can expect partner to take two or three tricks themselves. Your job is to contribute one or two and avoid blocking them. Specifically, do not lead trump when partner picks up: they have shown trump strength, and your trump lead just helps the opponents pull partner's trump.
Partner Passes in Second Round
If partner passes in the second round after passing in the first, their hand is genuinely weak in all four suits. They have no callable strength anywhere, which means you should not expect much from them in defense either.
If partner passes in the second round but called something or was being asked to call something, the message is more specific. A second-round pass when the call could have been the same-color suit as the up-card means partner does not have the left bower of that color (which would have been a strong second-round play if they did).
Partner Calls in Second Round
Second-round calls happen after the up-card has been turned down. The named suit is one of three remaining suits.
If partner calls the same color as the up-card, they almost certainly have the left bower (which is the Jack of the up-card's suit, now buried but still present in the color). A same-color second-round call is one of the most reliable calls in euchre because the left bower gives the caller a near-automatic high trump.
If partner calls a different color than the up-card, they have something else: probably both bowers of the called suit, or a bower plus an ace plus a second trump. This is a less common call but also reliable, because the off-color call means partner had strong enough cards to justify naming a fully fresh suit.
Either way, second-round calls indicate partner has serious trump strength. Your job is to defend partner's trump leads, hold off-suit aces for when partner draws side suits, and avoid leading trump (you would only be wasting partner's leverage).
Reading Partner During Play
Once the hand is in progress, partner's plays continue to reveal information.
If partner discards a side-suit ace early when not following suit, they are signaling either that the ace was a stranded high card with no path to take a trick, or that they want you to lead a different suit later. Either way, take note: the ace is gone and you cannot count on partner for a side-suit trick in that suit.
If partner trumps a trick you were going to win with your own high card, they have either misread the trick or are doing it because they have so much trump that "wasting" one is fine. Adjust your remaining play: partner is trump-heavy, so you can focus on side-suit tricks.
If partner plays low cards consistently on their first two tricks, they are short of high cards in those suits. Do not expect partner to take the tricks in those suits later.
What Partner's Bid Does Not Tell You
Bidding is information, but it is not perfect information. Partner could be playing a deceptive game, calling on a marginal hand to throw off opponents. They could be misreading their own hand. They could be playing under different conventions than you expect (an order-up in first seat is more aggressive than an order-up in third seat, but new partners may not adjust).
Use partner's bid as a probability shift, not a certainty. Their order-up makes a three-trick result more likely, not guaranteed. Your play should still account for the possibility that the bid was thin.
See Also
For the bidding decisions partner is making (and that you make on your own turn), see how to call trump in euchre. For the play side of the same partnership coordination, see how to play defense in euchre.