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Then they have to diagnose the question, "why is it using oil." This is normally followed by a compression and leak down test. Compression test will tell the condition of the cylinder head gasket, valves and cylinder rings. A leak down test will show what is specifically wrong in a cylinder if it fails a compression test. They apply high pressure to the cylinder via the spark plug hole, and where the air leaks out determines where the problem is.
If all are ok, then they have to look for a leak. Very seldom, but aluminum castings can be porous, allowing oil to leak out the pores of the aluminum. This usually shows oil streaks on the external engine surfaces.
In the early days of aluminum engines, this was not unusual. In the modern process, especially lost foam, it is unusual, but can happen.

If rings are bad, they will rebuild the engine, if valves are bad, they will replace the head, if gaskets are bad, they will replace a gasket. If the engine block is porous, they will usually replace the short block of the engine (engine block, pistons, crank, etc. but not the cylinder head). All depends on how they are servicing the engine.