I think most experienced dommes have a pretty good idea of what will fit with a submissive's stated interests.

If you tell me you're a sissy into strap-on, I'm gonna assume you also want to suck that strap-on and I'm gonna be right. Literally, I have never been wrong about it once.

Depending on the submissive and the activity in question, I might ask first but the times I've felt confident enough to go right on ahead without any check-in I have typically been correct. The few times I wasn't, the submissive had adequate opportunity to let me know before things went too far.