Water returns to well after pumping

Pump Experts,
The sump pump in my basement has been running constantly for about 2 months now - it pumps the water out, but as soon as it stops, I hear the water rushing right back into the pit through the pipe that was just used to pump it out. To me that sounds like the discharge pipe is blocked, but I don't know how to check it, or unblock it if that is the problem. Also, I searched around in the back garden to find where the discharge pipe actually comes out, but couldn't - I suppose it's buried. Is there a way to find out where the outlet end of the discharge pipe is without digging up the whole garden?

Thanks very much for any help you can provide,

Does the discharge line have a check valve on it? If not, put on in. If so, it's probably stuck in the open position and needs replacing.

That should be the problem solved right there....