You can test your diaphragm easily by sucking on that clear plastic tube attached to the cover (on the end of the injector pump). You should be able to build up vacuum quickly and easily (with your mouth) and feel the diaphragm hold that vacuum (if it's not torn).
I replace my diaphragm "in situ" but it IS tricky and you need a SMALL spanner (I have a tiny ring and open-ender set) and lots of care/patience.
It is easy to drop things inside if your not careful but I've successfully replaced mine twice now and I've never had the engine out of the vehicle.
(I did drop something inside during one of the diaphragm changes but I got it out successfully with a magnet.)
