I've been doing this or massage therapy since I was 18 and it's the subtle way people answer questions sometimes that let me know they are looking for more than just platonic... I wish we didn't have to do this but we do, so here are the main two I ask:
What are you looking for in your cuddling session? Or I'll ask what outcome they are looking for. If they use the word "release" I will generally dig into the word further. Sometimes they will also say they want to see what happens "naturally" this word is another red flag for me.
If I see that they have already been with another pro. I will ask how that session went and what was their favorite part. You can get a mix of answers here.
If they want to take the conversation instantly off of cuddle comfort it usually is a sign they want to ask for a sexual incounter. That has been my experience so far.
I always give people the benefit of the doubt that English might not be their first language or that they have trouble expressing themselves before ending the conversation but I am curious what other questions people ask. A lot of people who are looking for sex work know they can't just say it outright.
What are your red flags?