methods of detecting a copper pipe in concrete floor
This a is a very interesting question. I must assume, hopefully not wrongly, you are talking about a main supply line from the street. I can see no reason for an in house line to be embedded in the floor unless it feeds an out building or the like. In any case, an in-house line could be tracked back to some point of source and rerouted if necessary.
As for a street supply line, Mitch is correct in that the utility or a good metal detector can follow the line to the point where is enters the foundation. Usually it would follow a fairly straight path to the meter/main shut off from that point.  That would give you a good guess where the line is routed.  It will be very difficult to detect this line in concrete with rebar. The rebar will show on the detector and mask the copper. If you can find the point of entry, and follow it to the meter, use a placement for your sump pit far from this assumed line. As a precaution, I would be tempted to have the main water supply shut off while the sump pit is opened up. If you do hit the line, you won't have water gushing in at 70PSI.  If you're unlucky enough to find the line and damage it, then you can at least carefully work around it, clean up the line on either side of the damage and repair it in the sump hole.