peek: (noun) a quick look or glance; (verb) to take a quick look or glance, often secretively
Used in the expression “a sneak peek”.
peak: (noun) the summit; pointed top; highest level; (verb) to reach the highest level
NOT used in the expression “a sneak peek”.
A sneak peak is a mountain that is up to no good.
I think people often have trouble with this expression because they use the same vowel combination in peak as they do in sneak. But they shouldn’t. The memory trick I use is that peek has two Es just like the word eye does. And it is your eye that you use to take a peek. Another memory trick is that a capital A has a peak, so use an A in the word that refers to the pointed top. Let me know if either of those memory tricks will help.