The first Stud betting round, after 2 down cards and 1 up card are dealt.
The up card is the door card, and it decides who pays the bring-in. Action goes clockwise from there, with small bets. You can see only your own 3 cards and the other players' door cards, so at a 6-handed table, count how many of the cards you need are among those 5 (7 at an 8-handed table) before deciding to play.