The most important thing I have to say about this is - be sure to do it!
Keep track of everything, even if you think it's not useful to you at the time - it may be later on. Will you remember 6 craft shows later which location's customers really liked your new item and they sold out within an hour at a nice tidy profit?
In order to calculate how much, if any, profit you are making, you must track:
You simply must keep records or you'll never know where you stand. You may find your spreadsheet program becoming your best friend. Try to make note of every penny that comes in and goes out, and associate it with a particular product and, if applicable, particular sales method or location.
If it costs you five dollars in materials to make a lovely lace scarf, and you sell it for fifteen dollars, that sounds great! But if you're paying a sitter for each show you visit, plus booth rental, plus signage costs, travel costs, etc. etc., you may not be making as much money as you think.
That ten dollar profit can dwindle down to three dollars very easily. If it takes you six hours to make the item, you're paying yourself a grand total of fifty cents per hour! In this case, I do hope you're doing it for the love of the craft and not to pay your child's way through college. :)