Approximating calories in a donair you bought

A lot of big chain places have nutritional information online, so I always check the restaurant's website first for a nutrition pdf. Otherwise...

If it's obvious what is in the food I'm eating, I log each individual part. Like, if I have a reuben, I log two slices of rye, estimate the oz of corned beef, the cheese, etc. (And if it's somewhere I like to eat a lot, I save it in my meals for later use.)

If it's a dish that I can't guess the exact ingredients, I search for what I ate in the database and log a matching item whose calories seem believable. So if I order an asparagus frittata, I search "asparagus frittata", and might get multiple semi-matching results. I usually choose one in the middle of the range of calories; or if my meal seemed extra rich and fatty, one of the higher calorie content options. I err on the side of over estimating because I'd rather end up eating at a deficit than overeat.

It's not exact, but I eat out a lot and logging this way seems to work well for me.