The things to look at are:
the scale length - you want it to be 40" - 42" (at least)
bridge - you want it to at least approximate the shape and construction of an actual upright bridge
Fingerboard - like the bridge, it should at least approximate the curvature of an actual bass fingerboard.
And that the whole angle of fingerboard to bridge to tailpiece mimics a real bass.
You gotta understand that there are two different end users for these things.
1. guys who play a real bass and want something that they can fly with that will at least approximate the experience, if not sound, of playing their actual bass
2. guys who play electric and, for whatever reason, decide that they want to look like they are playing an upright bass.
Which is why you have alla the EUBs that are 35" to 38" in scale, have flat or only slightly curved fingerboards and have bridges that are nothing like an actual bass bridge. And sound like a big fretless bass.
The money that is being charged for the basses that I have seen in the former category, you could buy a pretty good plywood actual bass for. So if your buddy has one of these types of EUB and you're going to get it for a couple of hundred dollars, well OK. You go, girl.
If he has one of the latter, it doesn't matter how much he is giving it away for, it will not be an analogous experience. Buy it if you like the way it looks or the way it sounds or whatever. But don't buy it because you think it will be a "first step" or anything.
But I gotta say, even if it IS in the former category, if you want to play double bass, get a double bass. To me, doing something else is like saying "I want to paint my barn red. Maybe I should paint it blue, as a first step."
You want to paint the barn red, just go ahead and paint it red.