How to tell if brake pads are wore out

I took my front tire off now im trying to figure out if my brake pads are good how do i tell ?

Post a picture of them.

OKay now i really messed up i took everything off got the pads off and to me the pads look good i put a picture below could somone tell me if there still good or not but now my issue is how do i get the pads back in lol the rubber boot that goes over the piston came off how do i get that back on i took a photo of this also this is my first brake job i have the book and i followed it but i just dont know how to get the new pads in and also would like to know if these pads are good

No picture

its not letting me upload damnit

See image of new KZ650-C1 stock pads below. If yours are stock pads it’s easy to tell when they need to be replaced. They have a red line engraved into the pad; you may need to clean them a bit to see it. If they are not stock when the dead pad (one on right side in image) gets down to 1/16” or so or the live pad (one on left side in image) gets down to 1/4" or so it’s time to replace them. The live pad is recessed on the back side, so you cannot let it wear down more than 1/4”. Ed