When to retire aged copper extension cords with intact jackets?
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.