Actually, you can't.
Well, you can guess-timate some parameters, but there is no real way of finding specs without doing like CJ and completely disassembling.
What you can guess about is:
Weight to some degree tells you about VA rating
Secondary Voltage at no-load > is somewhat higher than at rated load. More over at small VA values than at large.
Secondary current can be guessed from wire dimensions, if visible
What you can't know:
- What primary voltage is supposed to be
- Rated secondary current
Jakob E.