Yes it is a lot of guesswork when eating out. I generally try and add up the carbs that are clear, so if I have a burger, I'd say the bun is ~40g carbs (depending on size), any chips (generally around 70g carbs) etc.
Most of the time its the burger fillings (or breadcrumb coated stuff) or sauces that catch you out, they can either be really high in carbs be laced with cheese or fats that delay absorption.
So you have to be careful how much and when you inject, I find that if I inject for most restaurant meals before the meal, then I will most likely have a hypo. Similarly I have to split my dose, so say 70% upfront then the remaining 30% an hour later.
High fat or protein meals generally have a delayed absorption, so the novorapid profile wont match that of the food, the novorapid will generally act by 3 hours in the majority, whereas the fats or proteins meals may peak at 5 hours. So you would hypo and then consequently hyper.
Also depending on where you inject makes a significant difference to how quick your insulin acts, if in the abdomen it acts much faster than in the leg for example. So injecting in the leg may have a better profile for delivery compared to food.
Also as above, I'd rather be slightly higher than hypo when out.