What tool do I need to check RAM compatibility?

CPU-Z is all you need. Open the SPD tab, pick a slot, and it shows the module’s type (DDR4/DDR5), speed, timings, voltage, and rank. Match those specs exactly—type and speed must match, timings should be same or looser, voltage must match. For dual-channel, the new stick should match the existing one’s rank. If unsure, buy an identical kit.