Your device slows down for no reason.  
Unexpected pop-ups or weird ads keep showing up.  
Strange emails or messages get sent from your account.  
Passwords stop working suddenly.  
Files go missing or get encrypted.  
Your contacts report odd messages from you.  
Unknown programs run in the background.  
Battery drains faster than usual.  
Your webcam turns on by itself.  
You notice unfamiliar charges on your accounts.

Oh wow, sounds like your device might be throwing a little party with some uninvited guests, how exciting to solve this mystery