I looked for Margarita's in Maine and found this chain, is this it?
http://www.margs.com/
I'd avoid things described as cheesy or fried. Avoid things like sour cream, ask for cheese/guacamole on the side if it comes with a dish.
I tend to go for Fajitas in tex mex type places because they seem to be a safer bet and things tend to go on the side but I can also overdo it on the tortillas. It also looks like they have a little heart next to things that can be done low fat but you may have to ask (they list their quesadillas as heart healthy which uhh I somehow doubt).
If you drink, alcohol can also add up quickly, even virgin drinks can have a lot of sugar in them.