Without any tests, how do I know that the series from 1 to infinity (3/5^k) converges?

I just wanna know if there's an easy way to look at the series without doing any tests (or if anything doing an easy test in your head) that the series (3/5^k) from 1 to infinity converges. I'm taking Calculus 2 and the book pretty much implies this is something you should know off the top of your head. Thanks.