Inspecting Potential Property - Analyzing Systems (heat/heater)

[Dewain J.]

As I'm walking through properties I'm potentially going to by, are there any tell tale ways to determine the condition systems (house heater, water heater, electric, etc.) just by eyeballing them? 

I know if I see knob and tube or a dated looking service that I'll likely need to update the electric, but what about the heaters?