Determining the rating of a transformer

On my quest to build the perfect guitar amp, I've dug this old power amp out of my closet. I was thinking about selling it to help fund a new transformer that will work in Rod Elliot's 100watt amp, but then I decided to see if it's transformer would work (it has to be a minimum of 25-0-25 150v 3amps).

On Rod Elliot's site, he has a great wealth of information on transformers.

I don't have the right equipment for all the tests that would be needed. Is it possible to measure the transformer at the rails with a volt meter? Or is there a simplified formula? Thanks

Well, measuring the voltage is only half of the pie.

You need to know how much current the transformer can handle before either sagging too much or heating up like miniature toaster oven.

If you're lucky, you may be able to lookup the part no. of the transformer. If not, you may be able to find some service manuals for the particular amp which would have more detail about the voltages and whatnot.
I should have done a little research before I asked about center tapped toroids. This is a quote for Rod Elliott's site

"Many transformers are supplied with two (or more) secondaries. In many cases, the data sheet will indicate that the windings may be connected in parallel or series. For example, a toroidal transformer may be rated at 2 x 25V at 5A (250VA). With the windings in parallel, the available current is 10A, but only for a single voltage of 25V AC. Connect the windings in series, and you get 50V at 5A, or by referencing the centre tap to earth, the familiar 25-0-25 designation."