How can I reliably check what type of RAM my motherboard supports?

Look up the exact motherboard model on the manufacturer’s website or a spec database like PCPartPicker. DDR4 and DDR5 slots look almost identical, but the notch is in a slightly different spot, and forcing the wrong stick can cause damage. Check the model in Windows using msinfo32 or look at the board itself for a printed number, then Google that plus “memory support” or “specs.” Don’t trust the price or seller’s filter alone.