how can i tell if the timing is off on a 2001 chevy suburban?

stop throwing parts at it, sometimes it is better to just pay the diagnostic fee to the dealer then determine whether or not to pay them to install the parts, ask to talk to the tech and have him explain why he suggest replacing the failed part or at least his diagnosis and have him show you test results

pull the #1 plug (driver side nearest radiator) and put a long wire or screwdriver in the hole with several inches extending out. Noe get a friend to slowly turn the crankshaft bolt at the dampner until the wire extends out of the plug hole to the max. Now shine a light at the dampner pulley from the drivers side top front of the engine. The mark on the pulley should align with the timing marks for 0 deg. If it is more than a few (5) degrees off then you probably need a timing chain . Check it twice as it is critical that the piston is as high as it goes

Check crank sensor and ignition control module.