How to estimate and verify square footage for a fixer upper?

[Olga Kostrova]

Guys, how do I estimate (well, actually verify my estimates) for the square footage of a fixer upper? Especially if it's a virtual wholesaling. 

I looked at similar properties in the neighborhood, but from photos it's hard to judge.

I cannot find data on any MLS services. It says zero. And neither seller not agent seem to know either.

[David Lee Hall, III]

Google Earth has a ruler tool. I use it all the time to measure building dimensions.