How do I check version of alpine mail client?

That works even without alpine installed (e.g. in Docker container):

$ cat /etc/alpine-release 
3.4.6

oh, just found it:

alpine -version

You should be able to run the command alpine -v or alpine -version ... you can also start Alpine and press ? on the main menu to open the main Help page, which will also tell you the version.

If your system has a package manager that you used to install Alpine, you can also check with that. For example, on my Debian system, I'd run:

$ dpkg -l | grep alpine
ii  alpine      1.10+dfsg-3     Text-based email client, friendly for novices but powerful