How can I tell whether an LS lifter is actually collapsed rather than just noisy?

A collapsed LS lifter usually gives you more than noise: a single-cylinder misfire (often with a cylinder-specific misfire code and check-engine light), rough idle and lost power. The valve on that cylinder isn't opening fully, and with the valve cover off there's often visible slack (a loose rocker) on that cylinder. A merely noisy lifter ticks but the cylinder still makes power.

On AFM engines the deactivation lifters are the common failure (Speedway, Michigan Motorsports). A lifter that doesn't "re-lock" leaves a dead cylinder even before you hear a tick. Left alone, it can wear the cam lobe, bend a pushrod or shed parts into the oil. A confirmed collapsed lifter means pulling the heads and replacing all 16 lifters (and the cam, if it's damaged or an AFM cam).