I haven't been doing this long but in the 6-7 years since I started repairing electronics I've yet to see a fuse blow just due to age, though I've heard people tell me that it can happen. Personally I would not put another fuse in there just to blow as this could potentially cause more damage. You could use a dim bulb tester, variac, etc., but you can also do checks for gross shorts. If you've checked all the rectifiers and main filter caps already, I would check the big power transistors in the regulator circuit, these are the ones attached to the chassis heatsink. I believe this would be Q1756, Q1766, Q1776, Q1788. Also, check the tantalum capacitors in the power supply, they can fail short.
I'm rebuilding a Tektronix 475A myself that has multiple issues and it has a very similar power supply.