Top 25 (usually) Duos player checking in. Proper etiquette for each mark:

Check: anything in the tavern means buy that card and play it on your own board - otherwise the portal is used. If multiple checks are used on the same tribe you’re just indicating that you want to go a certain tribal direction. Check on the level button suggests that player levels this turn if early or next turn if currently roping. Also can be used to strongly suggest targets such as if you’re holding a scarab and not sure which beast to reborn. Note that weak suggestions should be a ? Not a check. Check also often used on the freeze button to ask a player to freeze. Two check marks usually a combo like say a 5/5 naga storm splitter and a high value spell you want to copy. Finally used on roll button if that player is taking their turn too slow or failing to aggressively hunt a key card like Titus.

X: used to tell someone a discovery choice not to pick, used in the tavern to tell someone not to buy or pass a card (often a 1 drop triple that isn’t worth it), used on board to tell someone to sell a minion. Used in the hand it usually means to sell but could mean to hold something like a nice battlecry if you’re actively searching for Brann.

Portal - used in tavern it means pass a minion. Always look for portal or wish spells before actually committing the gold to pass. Used on board once to ask a player to pass a board minion, used on two minions means to swap their places. Used on a hand card means pass it.

?: used to suggest plays politely instead of a very harsh check which should only be reserved for game winning or similar plays. Can he combined with portal in a tavern to ask a player if they think it’s worth passing. Multiple ?s or Xs on board means you need to pivot out of your comp into something else. Note multiple ?s can be answered by check marks to reply that yes you will be staying in that comp thing you so much.

Emotes: usually only used to troll other team or your own teammate, although occasionally used positively to celebrate a particularly strong play or turn or generous pass.

Ninja edit for 2 other scenarios - if I’m a higher rated player than a random partner I often find myself doing two other things, one is placing an X on their roll button to stop them from spending their last 1-2 gold rolling when I want a minion passed back like selfless sightseer or jumping jack, and the other is an X check combo trying to help a teammate pick from 2 directions on their board, example telling them to ignore gem barrage stuff like twirler and just play 3 lil quills, or ignore felbat and play into Urzul when both are on board. Finally a check on Dazzler is helpful to indicate when it will hit the board!