Many (most?) of the device files aren't mountable. You don't mount a serial port, for example. If you are just curious as to what the /dev files are take a look at the devices.txt in the Documentation directory of the source code. In most distro's /usr/src/linux-<version>/Documentation/devices.txt .
Generally speaking, /dev/* are just "hooks" to hang device driver code on. The major and minor numbers have no real significance other than as a uniquely defined connection between the os and the kernel driver for the device it is an input/output for.
I don't know that there is any "automatic" way to find out which are valid in your system. Whichever hardware your system has (and has a driver loaded or compiled in) will be valid.