1999 saturn sl1 wagon has overheating problem. Had a leak in the reservoir so I replaced that. During that time had added stop leak several times. Reservoir is now new, but still losing water at speeds over 60. Radiator seems fine, hoses are ok, heater core not leaking, water pump is ok. How do I find out where water is leaking to?

If you cant see any water coming out anywhere then the only choice is a blown head gasket.

The reason the resorvior had a leak was the pressure from the engine got into the coolant and blew up the reservior bottle. This is a real common problem with saturns.

Get the car inspected. A simple test a a reapir shop will confirm this.

Be sure to flush all the stop leak out or it will clog the heater core up and the heater will not work.