A good place to start is by looking at the chain of narration of the Ahaadeeth and the reliability of each of the links in the chain (with Shi'a and Sunni using somewhat different standards wrt assessing the reliability of narrators). But we are all human and make mistakes, and Allaah never promised that She would protect the Ahaadeeth from corruption - only the Qur'aan. So we - all Muslims - should also look at how the Ahaadeeth measure up against Qur'aanic standards - interpretation, of course, and lots of potential for differences of interpretation and again, of course, error. Indeed, Allaah gave us brains to think with, and we should use them, and we should talk to others and consider their interpretations. And finally, most importantly of all, if we ask Allaah to guide us, She will answer.
Wrt the Qur'aanists, as with Muslims and of course the followers of other belief/knowledge systems, they're a fairly diverse bunch. One of the more prominent groups is that associated with Rashad Khalifa, who is believed to have declared himself to be a Messenger of Allaah. Since Muslims believe that Muhammad (s) was the final Messenger of Allaah, anyone who declares themself to be a Messenger of Allaah, and by extension anyone who follows such a person, will be considered heretics by most Muslims. Furthermore, all Qur'aanists to a greater or lesser extent reject the authenticity or authority of all or most of the Ahaadeeth, and the Ahaadeeth form such a central source of information/guidance on how to practice Islaam, so many Muslims might be inclined to consider them heretics on those grounds alone. Any Qur'aanists on here, needless to say, pitch in if you disagree with anything I have said.