Item for item, I've gotten into more trouble dismissing contributions as meant trivially when they were meant seriously than vice versa.

This is an understandable behavior, and I sympathize, but beware of overcompensating for it. Both mistakes are heavily penalized socially, for better or worse.

If you suspect, but are not certain, that something was meant humorously, consider replying in a way that can be interpreted either seriously or as carrying the joke further. Explicitly acknowledging either alternative is generally unwise.