There is something that is now troubling you about a guy that you have been talking to.
Perhaps you have had some conversations with him that have left you doubtful about whether he is straight or gay.
This may really be bothering you because you are worried that if you were to allow yourself to continue interacting with this guy, you may start catching feelings for him that may not be reciprocated on his part.
This is often the most effective kind of follow-up.
If these are old matches from an online dating site, you should avoid trying to send them some kind of interest signal as an attempt to see whether they are still interested in you.