Yes, I'm hesitant for this very reason, otherwise I would have probably purchased it already. I do not know the party selling the scope, and I'm suspicious by nature . I'm writing down all the things I would want to test before finalizing the deal, but it's hard to really check everything off the list. It would be pretty upsetting if I found out something was pretty off within the first 2 months of usage.
Well, fortunately, you can tell pretty much everything about the basic functionality of the scope with the probe calibration signal. Just using that will allow you to test the signal inputs, the triggering system, the acquisition system, the CPU, the display, etc., perhaps even up to and including the bandwidth of the channel. It's a 4 channel scope, and it's highly unlikely that the bandwidth of all channels would somehow be compromised, so if the signal looks the same (particularly at the corners of the square wave) across all channels then they're likely to be good to go in terms of the bandwidth. This presumes that the square wave itself has at least one transition that's fast enough to exceed the input bandwidth.
I have to second the notion of using the scope's self-calibration as a test, too.
If all of that works, then the only question left would be the digital inputs, the external trigger input, and the signal generator.
If this scope isn't being supplied with the digital MSO probes then you'll have no good way of testing the digital inputs, so the most you'll likely be able to do is a visual inspection of the pins of the socket.
For the external trigger input, you'd need a signal generator or something to test that.
And obviously, you can't test the signal generator unless it's enabled (but if it is, then you can test that directly and then use it to test the external trigger input).
Oh, and you'll want to test all the knobs and buttons on the front panel to make sure they work, and perhaps the USB connectors as well.
Anyway, testing the scope should be relatively straightforward, all in all.