What set is my Pokémon card from?

Three things on the card tell you

1. The collector number, at the bottom — something like 4/102. The second figure is the size of the main set, which on its own narrows things a long way.
2. The set symbol, next to that number, or bottom-left on older cards. Base Set is the famous exception: it has no symbol at all, so a card with none is very likely Base.
3. The copyright year, in the fine print. It tells you the era even when the symbol is worn or the card is in a sleeve.

Symbols are printed small — a few millimetres — and several are near-identical at a glance. If you can't tell two apart, the app reads the symbol and the number off the card for you.