Are there ball-check valves available for pex?
I haven't seen any direct-crimp check valves, but I have seen some "sharkbite" style spring check valves.  They take up very little room and are easy to install.



These will work on PEX or other types of pipes and can be removable if you think it has become inoperable.  I personally prefer anything with moving parts to be threaded or otherwise removable to make replacement or repair easier, but that's not an absolute rule.