I'm not sure how many others require this, or if it can be found elsewhere, but I often need to spell check a selection, not the entire document.
A couple methods:
1. select and hide everything else and hide the selection. Spell check only runs on what is visible.
2. Edit your text in a specific layer or sublayer, or just expand the layer to see the target text. Option-click (or Windows equiv.) the eye icon in the Layers palette. Everything else disappears. run spell check on the visible text.
Unhide or option-click the layer again to show everything else, and continue working.
Of course, this could be action based (select inverse, hide, spell check) and accessible via hot key.