Sure is an adjective.
- Waiting for hours is a sure sign of his patience and dedication.
Surely is an adverb.
- Waiting for hours is surely a sign of his patience and dedication.
Note: Careful writers try to avoid using the following informal uses of sure:
- Sure enough, I’ll be there.
- She sure is a nice person.
- He sure needed the money.
- Sure, I want to attend college.