lifters

How can you tell if the cam follower (lifters) is no good. Can't seem to find a straight answer. Please keep in mind they are hydraulic lifters not rocker arms. Thank you for your time.

2001 Saab 9-3 Turbo Base 2.0 DOHC 80,000 miles on engine Test drive; good. Buy car; good. Fuel up with 87 octane gasoline at 20 miles. 20 miles later in heavy traffic, noticed a distinct, metallic knock. We're now at 40 miles since purchase. ~100 miles, I suspect pre-ignition and add...

I recently repaired a lack of oil pressure problem that was caused by a clogged pickup screen, after taking of the oil pan and cleaning the components the car was running great, though a couple days ago the lifters, or hopefully one lifter began to start ticking as loud as it was before, then it...