In the vast majority of cases, the receiver handles crossover. The only exception would be if it's an older Pro-Logic or plain stereo model with no bass management.
If you're using a modern receiver with bass management, it will handle the crossover.
The sub has a knob on the back for adjusting crossover, which is used if you are using the speaker-level connections. This is the scenario where the right and left front speaker wires come out of the amp/receiver, go first to the sub, then out to the front mains.
If you use the sub pre-out and sub in on the sub (RCA pin jacks) you just need to set the crossover knob to a frequency higher than the crossover used by the receiver.
Most receivers have the crossover fixed at 80 or 90 hz, some let you adjust it.
Just make sure the sub's knob is set 20 hz or so higher than whatever crossover setting the receiver is using.