How do I buy a good used LS engine?

Buy as complete as you can. Speedway's junkyard guide makes the point that paying a bit more for the whole drivetrain — engine, transmission, computer, harness, sensors, even the whole front end — is far easier than hunting pieces later. This is especially true of Gen IV (about 2005–2014) engines, which depend on more factory sensors, computers and wiring to run properly.

Confirm what it is before you pay:

• Check the Service Parts Identification label (usually in the glovebox) for the engine's RPO code and the 8th digit of the VIN. From 2018 on, GM moved this to a QR code on the door-jamb label that you scan with your phone.
• Look the RPO up in our engine pages or the donor finder.
• Skip the LS4 (front-wheel-drive Impala/Monte Carlo/Grand Prix 5.3): its bellhousing pattern is for FWD transaxles and its compact accessory drive doesn't adapt easily to rear-drive swaps.

Then check its health — see piston slap vs lifter tick vs rod knock, and ask to hear it run if the seller can start it.