you can use a generic entry if it's something basic [club sandwich, ceasar salad, etc]. if it's a little more specific look for a large chain that has a similar item and use that as your base [if it's oilier or a larger portion it likely has more calories, smaller or more vegetables tends to be fewer calories] and adjust based on differences in composition and portion.
you could also try and break it down by ingredient [1/2 cup rice, 1/2 cup broccoli, two ounces of cheese] and track it that way.
basically you get as close as you can to it.
-google first. ask questions later.