For slow leaks:

If your bike has a flat, always check for a slow leak at the valve before removing the wheel. Put a little spit on your finger and wipe it on the end of the valve. Check if air hisses out.

Remove the faulty valve and put a drop of oil on the rubber gasket and reinstall the end of the valve.

Use the spit trick for Schrader valves to find a leak. See if a bubble forms. If it does, you’ll need to tighten the valve core.