Not a personal question but how big is yours?Two main things (beyond picking the appropriate size for your needs):
1. How flat is the top. The flatter, obviously, the better but if it's had some love and you don't need a precision instrument(!) then it's a good way to negotiate a discount. Mine isn't great, but only cost me £100
2. The bounce test - hold a hammer and let the business end drop onto the top of the anvil. It should bounce three or four times - if it doesn't and just thuds to a dead stop then the hardness has gone (or was never there) and you'll end up beating it to death
So I might be looking at a couple of anvils over the weekend, what do I need to look for in the way of condition? I'm not a blacksmith but don't want to end up with something that's only good for a garden ornament