You don't. You can check to see if any winding is shorted to another or to the frame. You can test for an open winding. Within some limits you can check for a shorted winding, but even if things check out OK with a meter, the transformer can still arc over at operasting voltages. The only real way to test the thing is to power it up. RG has a nice shorted windings tester on his site.
Just connect the tranny to the mains through a fuse or small breaker. Or bring it up on a variac while monitoring the current.
Education is what you're left with after you have forgotten what you have learned.
If your thinking you have a problem with the tranny because it's blowing fuses or not correct voltages on components your going to need to isolate the secondary windings from the load. Any faulty components will make the transformer read either zero or lower than it's nominal voltage. This isn't always an easiest thing to do on PCB boards but quite necessary. RG's site is http://geofex.com
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