You don't need a cap on them, but it may help keep them cleaner.
Behind that bearing buddy is a big castle nut with a cotter pin in it. You take the pin out, unscrew the nut and the whole tire/hub/bearnings and all will come off.
An easy check to see if they are ok is to jack the tire off the ground, spin the wheel as fast as you can by hand and put your head down by the hub and listen to it. If it is smooth and quiet its probably fine. If it has a growl or cyclical noise to it you probably need to give them a check and replace the bearings/races.
You have to pack the bearings before putting them in, youtube is your friend it will have all kinds of videos on this.