How to tell if a baseball card is worth money starts with the card rather than with a database. Four features, all printed on the card, predict value better than the player's name does, and one search confirms whether anybody is currently buying. Together they take under a minute per card.
A stated print run
A serial number showing the card as one of a numbered run is a disclosure about supply. It is the strongest signal available because it is a fact rather than an inference, and it is the reason to check a numbered card even when the player means nothing to you.
An autograph applied at manufacture
A signature on the card surface, described on the card as authentic, is part of the product and priced as such. A sticker autograph applied afterwards is worth less to most buyers. A signature obtained in person is a different market again, because no photograph can verify it.
Game-used wording on a swatch
Memorabilia cards state what the material is. Wording about use in a game means something quite different from wording about an item produced for the set, and buyers price them accordingly. The card's small print is deliberately precise here and is worth reading closely.
A date before the boom, and then a search
Pre-1980 cards came from smaller runs and survived worse, which makes condition matter and makes sharp examples scarce. Once a card passes any of these filters, one search of completed sales tells you whether there is a market. No sales means no price, and that is the honest answer for most cards.
Questions people ask about how to tell if a baseball card is worth money
Does a famous player mean the card is worth money?
No. The most famous players of the overproduced era have the most common cards, because demand for them is exactly why so many were printed.
Is a card in a hard plastic case valuable?
The case tells you somebody cared about the card, not that anybody valued it. Unless it is a sealed grader's slab with a certification number, it is storage.
What about cards that look unusual or misprinted?
Documented errors sometimes trade well; undocumented anomalies usually do not trade at all. Completed sales settle which you have.
Do I need to know the player's career?
Not for the sort. It matters for demand once the card is scarce and sharp, and by then you are reading sales rather than guessing.