If I am editing a .tex file in emacs, by default (for me) the bottom right corner of the frame will say "LaTeX/P". However, I won't get to that mode by typing "M-x LaTeX/P-mode"; I can only get to it by typing "M-x latex-mode".
Other than Googling it, how am I supposed to know the thing I type to get to that mode is "latex-mode"?
I think the simplest way is to check the value of the buffer-local major-mode variable, with either of:
major-mode RETmajor-mode RET
org-mode for example? (equal major-mode "org-mode") always return nil even when I'm in org-mode. "org-mode" is a string, whereas the value of major-mode is a symbol, so naturally they are not (cannot be) equal. (equal major-mode 'org-mode) would be t in org-mode buffers. Generally to test the current buffer's major mode you would use (derived-mode-p 'org-mode), which will return non-nil not only for org-mode itself, but also for any mode which was derived from org-mode. (type-of major-mode) to check that it's actually a symbol but not a string! C-h m gives you help on the current mode, and it typically tells you the name of the command that turns the mode on.
For example, in Emacs-Lisp mode C-h m tells you that you are in Emacs-Lisp mode. The command that turns the mode on is just emacs-lisp-mode.
C-h m also provides a link to the library that defines the mode, and if you click on that link it takes you to the definition of the mode command. For example, in Emacs-Lisp mode C-h m tells you:
Emacs-Lisp mode defined in `lisp-mode.el' And if you click the link lisp-mode.el then Emacs takes you to the definition of command emacs-lisp-mode, which is the command that turns the mode on:
(define-derived-mode emacs-lisp-mode prog-mode "Emacs-Lisp" "Major mode for editing Lisp code to run in Emacs. ...) It's possible to get the major mode just by evaluating this expression:
(print major-mode)
major-mode (emacs.stackexchange.com/a/18084/105).