There is not a lot of evaluation to do on handlebars - they don't wear out, so the only job is to determine if they have been damaged enough to be ruined. Any bar that doesn't suffer badly from one of the below problems is good.
Crash damage is usually obvious - if the bar is bent in anyway, it is ruined and should be placed with scrap metal (if it's hard to see the bend, you might want to bend it more to avoid someone using it accidentally). If you know an aluminum bar has been crashed badly and taken a good hit, it is good to err on the safe side and scrap it even if it shows no sign of it. Handlebar failure is extremely painful.
The most common kind of non-crash damage to handlebars is a "chewed up" look on aluminum bars from being twisted in the clamp of old stems (the kind without a removable faceplate - most of the bikes at the co-op have this style). If the chews are very deep, or if they're on a part of the bar that isn't reinforced (bars are usually reinforced in the clamp area, and usually in a very visible way), the bar may be bad. Ask a key volunteer or staff member to make the judgment.
If an aluminum handlebar has had a lot of usage, it is possible that it may have hairline cracks. This is not likely, but if you happen to see one, make sure that the handlebar is scrapped. Cracks spread quickly and catastrophically in aluminum, and this could be dangerous.