Is it possible to know the output Amps from a big power transformer?
The weight of a transformer can be a good indication of its power capability, it will probably be in the region of 25 Watts/lb (total weight including copper as well as iron).

Are you supplying 60Hz or 50Hz? - 60Hz has a higher capability for a given weight.

I would power it up and measure the secondary voltage.  The combination of the power capability and the output voltage should give an indication of maximum current output.

The gauge of the wire on the secondary should also give an indication of the current capability - there are tables of wire current capability versus gauge on the internet.

I found this discussion about this subject on another websiteAssessing an unknown transformer's capabilities.