[TCML] Clarification sought on transformers

[Stephen J. Hobley]

Hi,
 
I've been reading the list and noticed that there seem to be 'good' and 'bad' transformers for Tesla coils. I know someone with a neon sign shop, and he's got a bunch of old transformers laying around. What should I be looking for? 
 
...and more importantly, how can I measure it?
 
BTW - Big thanks to Dr Resonance for starting me out on the correct path last night - thanks for the phone call!
 
Steve
www.stephenhobley.com