I'm (hopefully) going to look at a used 2009 KLX250 tomorrow.
What should I be looking for, watching out for or asking about?

I'm handy with tools but have no experience working on or repairing motorcycles. I expect that to change but that's reality today.
Ed

 

Listen for rattling on the right side cam drive area when it starts it will be noticeable at 4000-6000 rpm, if bad it will be noticeable over more of the rpm range. Usually not a catastrophic problem in the short run, but a good bargaining point - you know, engine noise. The manual tensioner isn't that expensive and is easy to live with. I make them, have one in three of my bikes, one being an 09 KLX250, and a lot of riders here are using them. It is entirely possible the bike may already have one on it though.

Other than that I'd say ask about jetting, they definitely need it if not done and runs around $100±. Then the usual tires, chain/sprockets, and other common stuff.