There is a saying about people laughing so hard that it hurts. Perhaps... Laughing so hard that they cry.

Most people can't tell.

You tell by way of context. Say... They just got told bad news. Or they just got told a joke. The contextual cues should help you figure it out.

If you can't tell... The appropriate thing is to look... Quizzical. And a little concerned (ambiguous as to whether it is feigned / joking vs real)... And to ask them (in that ambiguous way) 'are you okay'?

Then they can clarify for you whether something is 'so funny' or 'so sad'.