These are fun ones and are very often misused. I think the easiest way to differentiate is to just write sentences with them.
EXAMPLES
Peek a boo! I see you!
The peak was over six thousand feet high.
The concept piqued the child’s interest, so they gave the child more to study.
We played peek a boo with each peak as it peeked around the corner; it was just enough to pique the toddlers interest and kept zir attention.
Adrian J. Smith 
This makes me think of Stealth Mountain. It’s a Twitter account, and the only thing they do is correct people who type “sneak peak” when they mean “sneak peek” (get it? :D )
They have 398,000 tweets already. I’d say that makes this a common problem.