If the primary and secondary windings have the same DC resistance, they're very likely to be 1:1, and hence an isolation transformer.

You can determine if it has multiple isolated windings (true transformer) vs it being an auto-transformer (like a Variac) using just an Ohmmeter between any wire connected to the primary and any wire connected to the secondary.

A transformer with isolated windings will show an infinite resistance. An auto-transformer will show a very low resistance...

 

You can determine if it has multiple isolated windings (true transformer) vs it being an auto-transformer (like a Variac) using just an Ohmmeter between any wire connected to the primary and any wire connected to the secondary.

A transformer with isolated windings will show an infinite resistance. An auto-transformer will show a very low resistance...

Yeah, I was in process of correcting myself, already. Missed the first time through that we were talking about variable transformers!

 

to3 and MileML got it for me. An isolation transformer has windings that don't touch each other...at all. The primary winding measured with an ohm meter to the output winding will show at least 500 million ohms (according to 1970 standards). A transformer that isn't isolated will show from less than an ohm to a few hundred ohms, depending on it's size where you measure. Quite easy to tell the difference between a hundred ohms and a hundred million ohms.