Pull all the tubes, turn on the amp and see if the fuse blows. If the fuse doesn't blow, then your power transformer is likely OK.
Next, put the rectifier tube in and try again. If the fuse blows, you want to have a look at the power supply caps.
If the fuse is still in-tact, add the driver tubes and recheck. While the driver tubes are in there, if the fuse is holding, check the grid pin of the output tubes (refer to the tube datasheet, you want G1) with your multi-meter to look for an appropriate amount of negative voltage. If you see something like -25V on one grid and 50V on the other, you have a leaking coupling cap that needs to be replaced.
These are kind of the basic first steps.
-PB