There are two main things to check for wear on a sickle:
1) Sickle section - depending on the type of section the following should be looked at starting with condition of the edge of the section. Check to make sure the edge is sharp, if you would not be afraid to run your finger along the cutting edge chances are the section is not sharp enough to mow hay. The section should be straight, not bent, and the bottom side of the section heel should not show wear.
2) Knifeback - should be straight and show no more than 1/16” wear across the back edge.