Is there any way to tell how much XP I've gained so far during a match?
From what I've seen, there is no progress bar about that. But you can notice how much XP you get for basic stuff like building smoething and do it every 4-5 minutes until you notice the drop then quit the game or finish the AI.
I'm pretty sure that just playing games after games will get you more XP than trying to force XP into a match. Level 30 arrives pretty fast!