I think a tell would be if they find ways to work tickling into conversation with people that they're attracted to. Like a lot of non-fetishists tickle as an excuse to touch someone so I wouldn't assume that someone who just randomly tickles people has the fetish, although some of them might. But if they bring it up, umprompted in a conversation where it's not relevant, then that kind of gives it away. If you look at the YouTuber Andre Meadows (BlackNerdComedy), I think you can see how obvious he is even when he's not outright asking the girls in his interview segments. Now if you know anything about this guy, you know that his tickle fetish is pretty well-documented and he's not trying to keep it a secret, but you can also see him bringing it up randomly to girls when it has no bearing on what they're talking about.
There was one video on his second channel, which I think may be gone now, where he was having a conversation with a girl about first dates. He was telling her about how he took a girl to an arcade on their first date and he said something like: "And I was poking her in the side while she was playing because we were both ticklish." That's a stange piece of information to include. If he had have just said he was poking her while she tried to play it would be one thing but including the 'because we were both ticklish,' made it pretty clear that he was trying to get this girl he was talking to to acknowledge tickling. Like he was either trying to get her to admit to being ticklish herself or perhaps even tickle him.
I don't think someone without a tickle fetish would notice this though. I remember having many conversations with this girl I liked over Facebook Messenger and being the awkward fool that I am, I tried to discreetly bring tickling into the conversation several different times. Of course there was never anything discreet about it and every time I did it I would think to myself afterwards that there's no way she didn't pick up on what I was doing. But then when I eventually told her about the fetish and asked her if she'd figured it out, she told me that she didn't have a clue.
So I don't know. I think some people can be very obvious about their tickle fetishes but maybe you'd have to be a tickle fetishist yourself to spot it.
EDIT: That BlackNerdComedy video I mentioned isn't deleted at all. Check out what he says at about 8:09:
https://youtu.be/LaWvVTEsr8o?t=489
He's being so obvious about it that it makes me physically wince.