In my personal experience, the blue food colouring method doesn't always work because you can't guarantee that the child has emptied their bladder completely. So if you want to know for the purposes of reward, it will work fine, but otherwise it's not super effective.
I have found that observing the child after is a much better method. There is a significant difference in behaviour before and after (in my experience with my daughter). After she's completely done, she is much more relaxed and comfortable, and be ready to move on with the day or play with her toys. If it was only a little (or none at all), she'll continue to do her need-to-potty dance, not want to move away from the toilet area, and be quite whiny and uncomfortable. This is definitely dependant on the child, what their 'tells' are, and that takes some practice to see.
It really depends on why you want to know, blue food colouring works for a reward, or just sitting on the potty is enough for that, but if you want to know in order to ensure you stick by the toilet a bit more or move on to something else, the kid's behaviour is the easiest to tell once you know what to look out for.
(As an aside, if you want to have a safety net for outings and aren't sure they've emptied their bladdar, use a light sanitary pad in their underwear. It will catch some of the pee, make them feel wet but still minimise the damage)