Baseball cards that are worth money are a small minority of any collection assembled for fun, and they are identifiable before you look up a price. The point of this page is to get you from a full box to a short pile in about twenty minutes, so the time you spend reading sales is spent on cards that might repay it.
Pull anything numbered
A serial number on the card is a statement about supply that the manufacturer is accountable for. Those cards go in the pile regardless of the player, because scarcity is the condition that has to be met first and this is the only case where it is stated outright.
Pull anything signed on the card
Manufacturer autographs are part of the card and trade as such. Check whether the signature is on the card surface or on a sticker applied to it; both exist, buyers distinguish them, and the sold prices distinguish them too. Autographs obtained in person are a separate market with its own authentication problem.
Pull anything from before 1980
Not because age is value, but because age is the best available proxy for a small surviving population when print runs were never published. Sort that pile again by condition before you search, because at that age the difference between a rough card and a sharp one is most of the price.
Leave the rest, and check the pile
Everything else goes back in the box. For each card in the short pile, find five completed sales of the same card in the same condition and put them through the checker. Twenty minutes of sorting and ten cards checked properly will tell you more than a whole evening of searching every card you own.
Questions people ask about baseball cards that are worth money
What if a card is famous but not numbered, signed or old?
Then it is probably common. Fame concentrates demand and does nothing about supply, and in this hobby supply is usually the binding constraint.
Should I keep the box as a set?
For common years, no. Sets from overproduced eras sell slowly and for less than the sum of the few cards inside them that matter.
Do I need special equipment to check condition?
A white surface and good light. A loupe helps for surface marks on modern foil, and nothing else is necessary to sort at this level.
What if I find nothing?
That is the usual outcome and it is not a failure of method. Most collections assembled for enjoyment contain no cards of significant value, and knowing that quickly is worth something in itself.