Markus,
It is possible to remove the injectors from the head and from the fuel rail and not damage the "O" rings on either end. BUT, it is a risk.
Just what is it that you want to inspect. everything is internal, other than the electrical connections.
It might not be a bad idea to have them cleaned and checked by someone who has the equipment to do that. However,I would not remove the injectors until I had a new set of "O" rings.
If I remember correctly, DANA makes an aftermarket set. Follow the other threads regarding the OEM "O" rings, and maybe you can get in on the "group" buy! rangerj