When they start to get wobbly when you move the wheel from side to side in the drop out then it is time to replace them.

Most of the ones I have are removed by taking the end caps off and tapping the bearing out with a drift pin. The first one generally comes out by tapping the axle with a plastic hammer.

For replacement I use a socket with the same diameter as the outside race of the bearing and just carefully tap it in. You can buy a special kit for pressing bearings back in.

Once the bearing is removed and cleaned I snap a picture of it with my phone and blow it up so I can read the identifying number on the bearing and just order some more. Seems like most of these bicycle bearings are made by a company called Enduro

The end caps usually have some kind of adjustability so you can put preload on to the bearings just tighten them enough so it doesn't wobble don't go nuts

You can't lube or service these bearings but put a little dab of Grease on the outer part of the bearing when you press it into the hub