If you played a game of word association with someone and said the word 'jeans,' we expect the first word you'd get back from them would be 'denim.' The second one would probably be 'Levi's.' That's the extent to which the brand has dominated - and continues to dominate - the jeans market. To the untrained eye, all jeans look pretty much the same, but if you're a collector, you'll be able to spot the differences between modern and vintage immediately. You'll also probably know when you're looking at a truly rare pair.
The ultimate find for a Levi's collector would be the 1950 'Anniversary range, which is worth $8,000 per pair. Even if you don't have that pair, check the logo on whatever you do have and see if the 'e' looks unusual. A misshapen or oversized 'E' is always a sign that the pair comes from a limited range, and is guaranteed to be worth hundreds of dollars online.