valid vs flawed argument

This is more of a general question, but I'm having an extremely hard time differentiating a valid and flawed argument. When there aren't sufficient and necessary terms and quantifiers in the question that allow me to diagram it, both the stimulus and answer choices all seem valid to me. I never feel like I have any sort of reason to question the validity in any of the answer choices for any question. Not sure if this makes sense, hope someone can help.