Determine what version and make settings accordingly

[Harry Putnam]

Probably kind of basic... but not to me.

I'm running emacs24 (installed from debian pkg system) and now want to
also run emacs-25... (installed from bzr repo).

I'd like to put something in ~/.emacs or even site-start.el (This is a
single user machine), that finds which version of emacs is currently
being started... and then set a few paths and etc differently than
usual, so as to load stuff from the --prefix=path of emacs-25.

Can anyone point me to some examples, that even with my elisp
illiteracy, I might be able to hack up and get to work for my
situation?