If not abused, do the conductors in extension cords degrade over time? If so, how does one determine when they need to be retired?
When to retire aged copper extension cords with intact jackets
extension-cord
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(note: this post assumes that by "gauge" you mean AWG)
I expect the biggest issue with longer leads is likely to be volt drop, especially during startup when motors can draw substantially more than their normal running current. Volt drop during startup may cause slower startup or even failure to start which in turn can lead to overheating.
I would suggest that if you need to use a longer cord you should aim to keep the resistance of the cord no greater than that of the cord the manufacturer recommends. So if you double the length you should also double the cross sectional area (which in american terms seems to translate to reducing the AWG number by 3).
Those cord reels are very useful. You'll need a BIG one to hold a 100' 12 gauge though. If you leave 6" or so hanging out one of the big holes on the side, you can wrap it with the end accessible.
I wrap these with the female end on the spool, so you plug it in first and walk to where you're working. I guess it works OK either way.
They make cord reels with receptacles on the spool ...
and those work great except when you're working on a ladder. Sometimes the cord on the tool isn't long enough to reach this thing on the ground.
When you're working on a ladder it's nice to be able to pull some slack off either end. This is a pretty good way to go:
That's from a tip sent into Fine Homebuilding.
The Cordwiz Pro
does the same thing as the bucket trick, if you'd rather not do it homemade.
One other thing to get if you're shopping, a portable GFCI
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Best Answer
If the insulation is fine, the wire is almost certainly fine.
If you plug it into a GFCI and it trips as soon as you plug it in (without anything plugged into the far end) it's toast.
If you see cracks, it's best to consider it toast.
If you plug in something appropriate for the gauge and length of the cord and it fails to operate normally (lights seem dim, motor seems slow) and does not exhibit those symptoms when plugged into the outlet directly, or into a different but still appropriate gauge and length cord, it might be that miniscule edge case that put an "almost" in the first sentence. Unplug the cord and feel for a part that's noticeably warmer than the rest - that would be where the rare defect case is, you can cut the cord on either side and have two shorter cords (for home, not commercial uses) or opt to throw it out. More often this is a problem of the plug or socket making poor contact, and the wire in the middle is still fine.
Until then, use it.