[Blue Bowtie]

Pressure testing is the customary method to locate leaks. That can be performed with a dedicated pressure tester or by starting the engine and allowing it to reach operating temperature. Shutting off the engine will maintain pressure for some time while making it safer and easier to inspect the suspicious areas, since the fans and engine would not be running.

What engine is in the Buick? Most likely it's the Buick 231 V-6, which has some typical coolant leak areas. If by some slim chance it's the Chevy V-6, it also has some (different) leak areas.