I'm pretty straight forward with them and mention it or even ask for extra sessions when I'm having such thoughts. Most of the time my T just asks what the content is, how bad the thoughts are and so on. Then we try to figure out what the actual issue is, since the thoughts won't go away without that being resolved. Two or three times he mentioned that maybe going inpatient would be good, but I'd have to agree to that in order for it to happen in my country, so it never happened, although at least one time it would probably have been a good idea.
Asking ahead sounds of time sounds like a good idea if you're worried. It also tells your T that this might come up, so she's already a bit prepared.