wait until it is warmer and then keep an eye out for nibbled leaves. they are often the cause.

wait until spring and then look on your cabbages ( cabbage white butterfly)

Make a noise like a nettle and wait for a butterfly to land?

Gosh ! I am amazed that you wish to find a caterpillar. Normally you see one day that your plants are half the size,and then with shock and tears,you spend hours trying to pick them off. I expect that spraying them when they are very young could keep them off,I seem to get caught every year.I am thinking of netting everything to keep the butterfly's away this time. I tried netting some of my Brussels and then I had white flies which were just as bad a problem.

If you still wish to find one,a bit later ,I could always send you one ?