What algorithm do you use to tell the difference between a feminist and a straw feminist?

I don't have a complete algorithm handy but I know that the first line includes the query "Does this person exist?"

It seems to me that any feminist suddenly becomes a straw feminist when an offensive or clearly irrational quote made by them is presented in a discussion about feminism.

It would be (arguably) legitimate to make that redesignation if the character is a fictional feminist. Then determining whether it is a 'straw feminist character' or 'feminist character' would entail an evaluation of to what extent the beliefs or behaviors reflect that which is within the realms of 'feminist', also taking into account inferences about the author's motives in creating the character. (Then you just shut up and say it is 'straw' if that makes your side of the debate look better.)