Check if the regular expression r made up of the single symbol alphabet Σ = {a} defines language L(r) = a*

Question

I have got to write an algorithm programatically using haskell. The program takes a regular expression $r$ made up of the unary alphabet $ \Sigma = \{ a \} $ and check if the regular expression $r$ defines the language $L(r) = a^{*} $. I am looking for any kind of tip, I think I can use a pumping lemma somehow, but I am not sure how exactly.