Value properties just by "looking" at them?

[Jonathan Feliciano]

Every now and then, I would hear a real estate investor say how they came across a 100K home, but knew it was actually worth 80K, or, I would hear the opposite: an 80K home actually worth 100K.

How do these investors know that---just by looking at a property---they can guess/estimate the property's actual value? Or in more general terms, how do they know if a property is over- or under-valued? What factors do they look at?

Looking forward to hearing your thoughts!