ispell-parser

ispell-parser is a variable defined in `ispell.el'.
Its value is

use-mode-name

Documentation:
Indicates whether ispell should parse the current buffer as TeX Code.
Special value `use-mode-name' tries to guess using the name of `major-mode'.
Default parser is `nroff'.
Currently the only other valid parser is `tex'.

You can set this variable in hooks in your init file -- eg:

(add-hook 'tex-mode-hook (lambda () (setq ispell-parser 'tex)))