How to find if $n$th root of $m$ is rational?

Question

I know that if $m$ is an integer, then the $n$th root of $m$ must be an integer, else it is irrational. But what if $n$ and $m$ both are decimals - is there a way of easily telling if the $n$th root of $m$ is rational or irrational without actually calculating all/a lot the digits?