Calling a method before execution of workflow

Hi all,

What I want to do is to call a method (or write a method somewhere in workflow) to check somedata (e.g. manager of some user, or maktx of some material, just a simple method) and check whether the workflow should execute or not using the output of this method.

I'm not sure where should I write or call this method in workflow. Anyone help me about this ?

Thanks.