Do you have previous experience truing wheels and building wheels? If not, then skip to the end, and perhaps verify your wheel is trued correctly before tearing it apart and rebuilding.

Are you currently riding the wheel?

If not, then disassemble now, and as FBinNY suggested measure it.

The way I do it is to measure the length of two nipples. Then put two spokes in on opposite sides of the rim, tie together with a rubber band, and measure from the bottom of the nipple to the bottom of the nipple, and add the lengths.

You can also lay the rim on a flat surface and verify that it is actually bent (although a hop might not be as easy to visualize).

You can also measure the length of your spokes and hub flanges. Note, the right and left are usually different lengths.