How to self-evaluate better? Keith Johnstone in Impro for Storytellers (Chapter 4, "Spontaneity") explains how people can assess themselves when practicing with another person:
After a scene in pairs I might ask the players, 'Was the work good?'
This confuses them, so I say, 'Your work is good if your partner enjoyed working with you!'
This idea is strange to them (many beginners have no idea how anyone else feels).
'Keep checking up on your partners to make sure they're having a good time. Think of it this way: if you're good but no one wants to work with you, I doubt you'll improve; but if your work is inept and yet everyone wants you as their partner, you'll soon be one of the very best.'
... [W]hen improvisers ignore their partners, I'll say, 'We're not going to praise you unless your partner is good.'