From your explanation, it seems that you're the one who brought up the "just be friends" topic first? There's a possibility he took that as a sign of rejection and just agreed since he can't force you to like him.

Still, if you want to give this a chance, just be direct and ask him about the situation instead of double guessing each other. I'm sure he'll appreciate it. There's nothing to lose, right?

Welp, case closed I guess lol! Best not to bring up the friendship topic the next time because that's like a very common rejection excuse and can be easily misinterpreted.

I don't think there's any foolproof way to know since everyone is different.

It's fair to drop someone if you're not feeling the vibe, but I feel a single date is a bit too early to tell, unless there's something that really turned you off? Some people may need more time to warm up before comfortable in showing their true self (the good or bad). Who knows!