Basically I try to make an informed estimate! Sometimes if you go to a chain restaurant e.g. pizza express you can look up online what carbs are in the food, or look up a similar ready meal e.g. look as a lasagne packet before you go out to eat one in a pub.
For other things, my carb-counting course at the clinic taught me to judge things by eye, e.g. 1 egg shaped potato is 10g carbs, 4 medium chips are 10 g carbs, 1tbsp cooked rice = 10 g carbs etc. So I look at my plate and add up all the carbs then I tend to inject at the end of the meal when I know how much of it I've eaten. It is more difficult to get it right when eating out, but you can always make a conservative guess then check your BG 2 hours later and take some more insulin if necessary.