It's a pretty good bet in most post war era US homes that the garage will have 2x4 walls on 16" centers, as a minimum, if there is adequate sheathing (plywood, chipboard or diagonal board ) it will satisfy most locations without seismic concerns. In my region, a lot of local inspectors will make you excavate a small area of the supporting foundation wall and footer to gauge if the existing is adequate to add more load to. Since a lot of foundations predate current code requirements, this leads to a lot of unhappy homeowners, since many builders would cut corners with shallow undersized footers, and walls that were too short to provide frost protection. Nothing to find 3-4 courses of block on top of a 4" thick footer in my area, when six courses and an 8" thick footer is what the inspector needs to see. Bottom line is, once you you know that it's legal to proceed, for many the next step is a shovel to find out if the original structure is built on a foundation worthy of a second floor.