u gonna need sensor
https://modmymods.com/phobya-2-pin-temperature-sensor-extension-80cm-black-71223.html
if your mainboard has temperature headers, you can connect it there, if not, then u will need some controller which can read it
plenty of fan controllers with display support temperature sensors, but they need front bay...which most cases doesnt use anymore
this could be feasable option, iit has its owns sensor...so no need to purchase another (unless it breaks)
https://www.amazon.com/Coolerguys-Thermal-Monitor-Digital-Display/dp/B07F46D8ZC?th=1
now you just need to slap that sensor directly on one of memory chips, place it between memory ic and thermal pad