[RobertGT]

I wouldn't go far as to say they have all been tampered with. There is still plenty of legit unopened wax out there. The key is to buy from a reputable source. Steve Hart of the Baseball Card Exchange is certainly an authority in this field and a good place to buy from, but expect to pay a premium for his packs. Since you are looking for 1951-1975, you are going to put out some decent cash anyway. Keep in mind even Steve Hart has been duped a few times, and I'm sure the board member Pepis will chime in on this later.

If you want to go it alone, I would recommend buying Mark Murphy's (The Baseball Card Kid's) unopened pack guide before you begin your quest. I think you can still buy them for less than $10 on eBay. Mark Murphy, in my mind, is still the foremost expert when it comes to unopened wax, even though it seems he has almost dropped out of the hobby as of late.

A quick primer:
-Look for tell-tale factory roller seals with wax on the back of all Topps packs. This is a good thing - a sign you have a good pack with factory rolls intact.
-Check corner folds on the back of wax pack. Each should have a factory fold at a 45 degree angle with a tight seal, just like a present. Most reseal jobs are easy to spot just by looking at the sloppy condition of the pack.
-Tears or holes in the corners of the packs. Likely the result of refolding the wrapper. Wrappers are fragile and will break down easily after a couple attempts to refold.
-Beware of signs of excessive heat on the wrapper, i.e. brown "burn" marks. Likely a sign of a fraudster using an iron to reseal the pack.
-Glue on the pack, or pack is to hard to open/tears wax paper when opened. Resealed.
-No gum that you can feel within the pack. Pretty obvious here.
-Rack packs - Notches/fingernail marks that leave an indentation on the top of the card. Someone attempted to pull the top cards back through the pack to reveal which are underneath.
-Some racks and cellos can also be sequentially searched depending on which player is on the top or bottom. Nothing you can do about this. Pack is still legit, just may not contain any stars or RCs.

Hope this helps and good luck.