Welcome to PokéCommunity! Register now and join one of the best fan communities on the 'net to talk Pokémon and more! We are not affiliated with The Pokémon Company or Nintendo.
To check whether a nautre is beneficial or hindering you can identify this in game by looking at a Pokémon's stats, the one highlighted in blue is Hindering, the one in red is Beneficial -- This is only Generation four onwards IIRC.
And that answers that! Please use a Q&A thread next time. There's one in each specific game forum, so assuming that this was for Black and White, you would ask there for example.