how do I no when the detents need replacing as still a novice at the sport when it becomes technical thanks Ash
They aren't something you need to replace regularly.
Easy way: Take barrel off and feel with your finger in the breech, the ends of the detents,
they should bristle/feel firm as you run your fingers over them and if there is a lot of play in them replace them.
Longer way: Take of eye covers and take out detents do the same as above and look for any splits or tears