When cannabilising from other gear, this can be a little bit of a challenge.
The first step is to check that it does, in fact, connect directly to the mains. This will confirm the primary is designed to handle the voltage. As such, it will be suitable for a linear power supply - but whether it will be suitable for the power supply you want, is another matter.
Getting the secondary (or secondaries) voltage is not too difficult - just wire it up and measure the voltages. Finding the current capabilities is a little less straightforward. If you can get the specifications for the transformer, then these questions are immediately answered.
Usually, there is a correlation between physical size and weight with voltage and current capabilities - the bigger they are, the more grunt they can deliver. The trick is to have the right voltage(s) on the secondaries, with current capability being equal to or more than you need.
Buying a transformer new makes this trivial.
As always, when working with mains power .... SAFETY FIRST.