I got an email from google on Feb 29th that I have some possible invalid clicks. I did not get banned, but I have no clue what they're talking about. How can I find out exacly which traffic of mine they are considering invalid? Seems like they're telling me I'm guilty with no proof. How can I find what is bad?
install third party softs to check invalid activity . consider it as your last warning after that they might ban you
Check your logs, you are lucky to get a warning first before being banned. BTW they definitely have proof before warning you, they are just giving you a chance to correct it before they ban you. So check carefully.
You should check your traffic sources. May be a huge traffic comes to your site with a particular network and get clicks without any aim. It is not possible that Google Adsense team warn everyone with a specific reason before ban, if you got any warning need not much worry, try to find the reason by analyzing traffic, problem will be resolve.
I would recommend logging in regularly, to check on any strange increase in click throughs, report any suspicious events to Adsense, in this way you keep yourself safe just in case any click bombers try to get you.