I install keyloggers on all my friend's computers to find what porn they search for.
But in all seriousness, I find that people who are normally very comfortable with comedy, able to roll genius jokes out with barely half a moment to think, tend to be more likely to trip up or overthink a joke that is relevant to their fetish. In fact, they'll often spend a lot of time pondering whether a not a joke is appropriate or not within the context of their own thoughts, even though it's completely innocent to everyone else in the room.
More reserved, less fluent comedians, however, seem to have an opposite problem. Their fetish-related jokes still tend to be a bit awkward or stilted in delivery, but they stand out among the others.
It's definitely not certain, every comedian stumbles through a delivery every now and then, and even the most awful of jokers occasionally hit one out of the park. I'd typically look for patterns and do some subtle probing from there.
As for fetishes that are shared with me personally, I like essentially say "Does the following ring any bells", then say a sentence filled with keywords that, taken out of context, are vague enough to be meaningless to anyone outside the community, but specific enough that, you know, it leads to your fetish and few to no others.
Here are some keywords/phrases I've found good for hypnosis:
"Melty"
"Go/Put under"
"Relax to let go"
"Talk you/them to sleep"
Obviously, again, not 100% reliable as smaller, self-contained communities may not practice the same diction, or the person you're talking to might just be a bit... slow.