Computer
Fault codes, live data, active tests, adaptations. It shows what the control units see; on its own it does not name the faulty part.
Borescope
A camera look into the cylinders without dismantling the engine. It shows surface condition but does not replace measurements.
Smoke test
Finding leaks in the intake and vacuum lines. Suited to air leaks; it does not show mechanical wear.
Compression
Comparing pressure between cylinders: rings, valves, head gasket. The result is read together with other data.
Oil pressure
Checking pressure in the lubrication system helps separate pump or wear problems from a faulty sensor.
Cooling and timing
A chemical coolant test for combustion gases when a head-gasket leak is suspected, plus inspection of the timing and cooling system: water pump, thermostat, hoses and timing drive.