Most likely they're going to be prohibited from telling you. The best way to do this is to take [person] out for a round of beers. Wait until 3 beers in and see what their number is.
My belief is that 20%-30% of what you're making is a fair number.
Though if it's somewhere where you could really get a foothold and make a lot of money as an employee, 50% might be acceptable.
