I read about valves that have removable cores and that is better for applying sealant. How can I tell if my valves have cores that are removable? (I don't have the type of the tube, or any other info; used bike).
Have a close look.
On a presta valve there is a wide threaded part that the lock ring attaches to. And a narrower part with the valve lock nut above it that the cap threads onto.
If the narrow part has two flat sides to get a tool onto, then it's removable.