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I vote for just replacing all caps before you use it...there are not that many caps in there. Usually the filter caps in the GE's are bad. Low voltage is not good for AC motors. Usually I just plug it in if it is from the 50's or later...have never had any trouble except a line bypass cap shorting in one set. I then will replace all the caps before I really use it as most of the time at least the electrolytics are really bad and replacing the paper caps yields better performance.
If it's really old stuff like from the 20's-40's all caps get replaced before it even gets plugged in as there is a higher likelyhood of shorts in this really old equipment.
When a short is suspected (like when a fuse has blown) I use a 60 watt light bulb in series with the power line to limit the current and help diagnose the short.
Sometimes in tube type electronic organs where there are hundreds of caps I have just left the old caps in, if it's my own equipment and it appears to be working OK. I'll use the ESR meter to help spot electrolytics which are going bad if I elect not to replace them all.
In many hundreds of devices I have worked on, from organs to amps, radios, projectors, etc. have never used the variac to slowly bring up voltage...I just follow the above guidelines. I know some people have got success with it, though.
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