Flyspell enables on-the-fly spell checking in Emacs by the means of a minor mode (called Flyspell).
Flyspell is compatible with (La)TeX editing. That is, Flyspell tries, as much as possible, to avoid highlighting TeX command. In order to automate the starting of Flyspell when editing LaTeX source files, one may add the following expression in his .emacs file:
(add-hook 'LaTeX-mode-hook 'flyspell-mode)