[racket] Detecting cycle in directed graph

I have a directed graph given as list of pairs ((from-node .
to-node) ...) and need to detect whether adding a pair would create a
cycle.

Is there an easy way to do this already, e.g. some Planet package or
snippet someone would like to share? 

I don't want to reinvent the wheel and this is not a homework question.

Best,

Erich