jardeko

Jar + dek + o: a ten of years. Decompose it, check that the meaning comes out right, and it does - a decade is exactly a ten of years. Deko is a noun made from the numeral dek in the ordinary way, the same move that gives cento a hundred-of and therefore jarcento, a century. Learn the pattern once and jarmilo, a millennium, needs no explanation either.

Now put it beside ferdeko, and the whole method of reading Esperanto compounds is in the pair. Ferdeko is the deck of a ship. It ends in the same five letters, -deko, and in front of them sits fer-, which really is the Esperanto root for iron. Decompose it the same way and you get "an iron ten", which is not a thing. So the decomposition is wrong: ferdeko is a single borrowing of German Verdeck, and the dek inside it is an accident of spelling.

That is the test, and it is the only test you need. Split the word, read the pieces, and ask whether the meaning that comes out is the meaning the word has. A ten of years is a decade, so jardeko is built. An iron ten is nothing, so ferdeko is not. Esperanto will not tell you which is which - the seams are invisible - but the meaning always will, and that is why the check is worth making a habit rather than a special effort.