Motivation

Difficult without knowing all its components and being cognizant of the portion sizes.

Your best bet is to look up the restaurant and dishes therein before service, overestimate the cumulative calories and underestimate the calories burnt for that day for a comfortable margin.

Your odds of accurately estimating the calories in restaurant food is low, unless it's a small, simple dish. I'd try to limit buying complex items from restaurants that refuse to share nutritional data, or find a comparable items in the MyFitnessPal database and add +25% to the portion.

Some interesting data--

Anyone's Guess by CSPI wrote:

A study conducted by the Center for Science in the Public Interest and New York University found that even welltrained nutrition professionals could not accurately estimate the calorie content of typical restaurant meals (Table 6; Backstrand et al., 1997). Although the dietitians were able to accurately estimate the caloric content of a cup of whole milk (the control in the study), they consistently underestimated the calories in restaurant foods and meals. Their estimates were off by large amounts – by 200 to 600 calories. For example, when shown a typical dinner-house hamburger and onion rings, the dietitians on average estimated that it had 865 calories, when it actually contained 1,550 calories. Since not even experts in the field of nutrition are able to accurately estimate the caloric content of restaurant foods, consumers are unlikely to do better.

My big shock in this area came a decade ago when it was revealed a popular restaurant's "healthy" dish--grilled chicken--contained more calories than their bacon cheeseburger. Their goal was apparently to make the chicken seem healthy while being very tasty to boot, so it sold well. Actually making the chicken healthy clearly wasn't one of their business priorities.