If the paints weren't opened and have not been exposed to extremes of temperature, there shouldn't be any reason why they shouldn't be useable. I have several jars of Testors paints which have sat unopened since the early 80's, if not earlier. When shaken, they will mix up just as they did when I bought them, so I am presuming (without opening) that they are ok to use.
Having said that, the best way to be sure is to open a jar and stir with a suitable implement. After stirring for about 3-4 minutes, if the paint appears smooth and there are no lumps or strings that will not mix in, then your chances are looking better. If you do have strings or lumps of congealed paint which won't mix, bin it, as this is generally unrecoverable.