Western hercules beetles on ash; note male horn on top and size difference. Photo by Alex Wild.
Depending on how good your eyesight is, you will be able to tell male from female Japanese beetle. Look at the first pair of legs and focus on the tibia. The tibia is the late large segment before the tarsus, or "feet." You may need a hand lens to see the tibia. The male will have spikes on the tibia and the female will have more spoon-like paddles.
Male Japanese beetle. Photo by Tom Hillyer.
Female Japanese beetle. Photo by Tom Hillyer.
Or just use the squish test to look for eggs in female Japanese beetle. Photo by Tom Hillyer.
For more information (with drawings), visit these websites: University of Tennessee Extension and NAPPFAST.