I'm going to correct myself; you can preview the video and the chapter marks in Pro Tools. You can't preview the menu operation. There is a limited menu preview available in DVDLab (it shows what will happen when you select the various menu options), but there isn't a full-function preview available if that's what you were referring to.
What I do for complex menu testing is I build the VIDEO_TS directory using a built-in option that replaces the actual videos with blanks. This lets me quickly generate the entire menu structure. I test it using a 3rd party DVD player (for simple menus you can use a freeware player, which includes WMP and Vlan's VLC player). Then I build the full DVD and test it before burning.
In general, unless you're doing something complex (and by that I mean programming the DVD engine manually) the menu simulation feature in DVDLab is good enough to show you how things will work. IMHO of course.