If this is one of the main reasons you're collecting knives, i would suggest another hobby, but for the other part, looking for an aditional value of collecting... you've found the right place! There are a few key-ingredients for a valuable pocket knife. Here we go....
*Genuine Victorinox? : Look for the Victorinox/Elinox/Elsener/Victoria logo at the main blade! Don't be satisfied by the swiss logo on the scales! This is ALLWAYS a forgery if the main blade inscription is missing. Once you open the knife, an you know the quality of a real one, you'll feel the difference!
*Main blade/small blade : Look for amateur-sharpening. You'll notice this when you look at the sharp edge. If it's a millimeter of scharpened edge, it'll probably be ok, but when there's more, and the edge is scratched, there's a chance that the blade is shortened by years of severe sharpening.
*Scales : Are the scales damaged beyond repair? Cracks can be repaired, but it's difficult! Small scratches can be easily repaired!
There are a few websites (look right at the links-section) with detailed info for dating your Vic. A few things make it more valuable.
*Collectability : Is it a limited run, a damast edition or part of a wanted collection?
*Age : The older its gets .... It isn't allways true, but mostly this rule can be applied! Look at the condition.
*Fool's money : We have a clear rule in Belgium... It's worth wat the fool is willing to pay for it! Find someone with the right interests and he'll buy it. Don't exaggerate! Everyone has his limits...
*Ebay : Look at Ebay for a reference. There are a lot of Vics out there and it's easier to see wat others are willing to pay for it!
I hope this gives you an idea...