It depends a bit on the type of train you were on but usually the seats look more luxurious and are a different color, it are smaller cabins with doors and most importantly there's a big fat 1 on the wall, usually on the outside of the train too.

Oh, and about 90% of the seats are empty instead of there being people standing in the isles because barely oneyone travels first class.

The chairs in 1st class are always red. And there is a number "1" on the wall on the inside of the compartiment and right on the outside of the train.

The red chairs are somewhat wider than the 2nd class blue seat ones, but that's about the only difference. People using 1st class in general pay the higher fare for a quieter travel experience and an almost 100% chance having a seat.

As 2nd class often is full and loud (even in the quietzone!) 

This post is not very believable. 1st class is clearly marked inside and out. 

When you are sitting in the first class with a second class ticket you are not paying the right amount and you may be fined. It doesn't matter whether it is on purpose or not. It mostly is, but even that does not matter. The train conductor took the right decision.Be honest and pay the right amount. You were sitting there so you enjoyed the first class and you have to pay for it.