1966 Mustang coupe. 289, 4 bbl. Basic stock, nothing fancy.
I have a tapping, clicking sound from the driver side of the engine. A few different mechanics listen to the sound and speculate it’s one of the lifters. They say replace the engine. I am not convertible automatically spending a minimum of $5-6K on an engine rebuild or crate engine.
What specific tests or diagnostic steps can be taken by a mechanic to better diagnose the root cause (rather than simply relying on a sound)...

Engine makes tapping/clicking sound