You can start with a DC R measurement. If they seem fairly precise at DC, that's a good sign that they were intended to be so there and beyond. The RF/HF is of course a different story, but good LF/DC is necessary before you even worry about going up. Most typical terminators that I've seen are around +/- 5%, so nothing special. if you see say, one percent or better, then maybe they're pretty good - at LF/DC at least.
You can check them at RF/HF too, if you have the right gear. The type/grade of connector is a big factor too - if you're talking about BNC only, it's not a big deal.
Ed
