If you can take it out at night and run it for half an hour to an hour and make sure you don't have any issues with it running, I'd say if it's a good deal & looks nice, take your chances and go for it. Anything can fail, but if well cared for & you don't leave the batteries in when not using it, chances are you'll be fine for a pretty good amount of time.
How much are you looking at? Have you looked at TNVC, called them? I have a hand select OMNI8 ITT unit and that's about the best way you can go if they're still available. The phosphor ones I found I like better, I think they have no film? If I get another set, it'll probably be those in a dual aviator setup but that could be quite a while before I'll ever be able to do that.
Not sure about Morrovision, I only do business with the mfg. or TNVC, they make their own inhouse line that's also a good pick. I'd call 'em before making a decision. They really know their stuff and won't blow smoke up your ass or pressure you into a sale. Great folks.
Among the things that I would check with a used PVS-14 would be to make sure that there was no significant damage to the lenses, that both objective and diopter focus had their full range of motion and rotated and functioned smoothly.
I would make sure all of the functions worked properly, including the IR illuminator, and the manual gain, and if you’re able to put it in a mount, even the auto-shutoff, though I usually disable it in practice—it could give you a good indication of whether or not it’s been properly built. The auto-shutoff isn’t always necessarily reliable in field conditions, but it should work under “ideal” conditions, but it also requires the J-Arm and mount to be in full serviceable condition as well, which is not always easy to tell with used/surplus parts, because again, some users will disable them in various ways.
If possible, I would also try to look through the rear eyepiece and try to record the information off the tube, many tubes will have information visible at the right angle through the eyepiece without needing to disassemble the device.
I would also try to see if I could dectect a faint, high pitched whine which would indicate an auto-gated tube (that being said, just because it’s not auto-gated doesn’t necessarily mean it’s a bad unit, especially depending on price).
Other things would just generally be quality of assembly and condition of the unit in general, everything is tight, no cracks in the housing, etc.
To be honest, when buying used night vision, I will often rely as much on the person I’m buying from and how well I know them and their reputation, and whether or not they seem like they know what they’re talking about. Lots of good deals to be had on used NV out there if you know what you’re looking at, but very easy to get burned if you don’t.