Correct way to verify square footage?

[Bienes Raices]

I'm making an offer & trying to verify what's in the MLS. The house has a detached garage, so that makes it easy to exclude garage. When I measured the individual rooms in the house and added together, it came up short of what the MLS listed as square footage for the Living area. Am I supposed to be measuring outside the perimeter of the house instead, or is the MLS/appraiser site wrong? Thanks.