[sjs382]

You can't know. GA spam is rampant, more so via referer spam than anything else.

[Mahn]

This is the correct answer. You may be able to filter data to some extent in the dashboard using special views, but if you want a 100% guarantee that your data stays reliable, currently your best bet is not to use GA, or at least complement it with another tool.

[johnward]

Referrer spam is the worst. This was on a reddit thread today: https://i.imgur.com/mRGiiBQ.png

note the: how to stop referral spam url