Baseball cards that are worth a lot of money are not a list you can memorise, because the list changes and because the same card in two conditions belongs on different lists. What does not change is the pattern: they are scarce, they are sharp, and people are actively buying them. All three, at once.
Scarce in a way that can be shown
Either the card states its own run, or it comes from an era whose surviving population is visibly small because examples come up for sale rarely. A claim of scarcity in a listing is not evidence. A printed serial number is, and so is a sales record showing three examples a year rather than thirty a month.
Sharp enough that condition is doing work
Among scarce cards, the price ladder between conditions is steep, and the top of it is where the large numbers live. This is why the expensive examples of old cards are overwhelmingly the graded ones at high grades: those are the survivors that escaped a lifetime of handling, and there are very few of them.
Wanted by enough people to make a market
A card needs competing buyers to reach a high price, which means the player, the set or the design has to matter to a community rather than to one collector. Scarcity without that produces an object that is hard to sell rather than an expensive one, and the sales record shows the difference immediately.
How to test your own card
Check for a stated run or an early date, judge the condition honestly against a white background, and search completed sales. If all three come back positive, price it properly with five comparables and consider whether grading would add more than it costs. If any one comes back negative, you have your answer and it took a minute.
Questions people ask about baseball cards that are worth a lot of money
Is there a list of the most valuable cards?
Plenty exist and they are records reporting rather than a method. This site publishes no such list and no prices, because a name without a condition and a date is not a price.
Could a modern card be worth a lot?
Yes. Low-numbered parallels and manufacturer autographs of players in demand reach high prices, sometimes quickly.
Does grading make a card worth a lot?
It can reveal that a card was already at the top of its condition range. It cannot create scarcity or demand, which is why most graded cards are not worth a lot.
What is the single best predictor?
A stated small print run, or a sales record showing how rarely one appears. Both are facts about supply, which is the constraint that binds.