Inspection for mice problems:
Learn how to investigate a mouse problem in a structure at Inspection Tips
Exclusion:
Tighten the building. As long as openings aren’t being used by insects or animals larger than mice, secure all of them ¼” or larger with caulk, ¼” sized hardware cloth or flashing. Use caution around weeping vents as they need to remain open to drain water. Use aluminum mosquito netting to stop entry but still allow air and water flow.
Additional info is found at Prevention Tips
Exclusion also includes removal of harborage (piles of stuff and long grass) as well as bird feeders. As food increases so does mouse population.
Resources (online)
Rodent Exclusion
