Hank you're asking a difficult question. You have here in general 2 types of models: ones more "typical" which are generally the classic ones: aircraft, tanks, vehicles and ships.
Specially if they're are done by a designer for selling they have an scale: 1/33, 1/48, 1/50, 1/72 ... you can have a look here:
http://www.ecardmodels.com
Look there for the designers like Kamppflieger, Thaipaperwork, Marek, Jacek, Krychu74 and others.
If you see in the forum a model from one of this designer probably you can know the scale.
But in the papermodel community you have a lot of models which are done out of scale, generally from movies, TV, imagination and that models are difficult to know the scale.
In that case you have to look for a picture with some object to see the scale: the typical scale cubes, or just a hand or an stick in some cases like this one:
Which is my build at 1/72 scale model of 3 ton ford tank from wwi by Kamppflieger
The other subject to pay attention is the fact is very common in cardmodeling to rescale models. A lot of people for example prints a 1/48 model at 67% and build it at 1/72 scale, so in the scale is relative.
I know this isn't a direct answer to you question but I hope to help you to enjoy this amazing hobby.
Regards
Alcides