Well, you want a really good looking one with a nice shape... When ya hold it, take a whiff of it and make sure it doesn't smell too fishy...
Also, make sure the holes aren't obstructed by anything.. Sometimes they get crud or something that obstructs the flow and could cause problems... If there's any crud, it can be cleaned, but ya have to wonder how well it's been kept up until the time you've found it....