It all depends on how much time and effort you want to put into it, and how quick you are with a calculator.
I do estimate things like that burrito bowl... if I think I ate "about a third" of something, that's what I go with. I figure sometimes I'm probably guessing low and sometimes I'm guessing high, so it will even out eventually.
For the homemade stuff, if you have the recipe, you can calculate it yourself. Add it all into a calorie counting program, ingredient-by-ingredient, to get the total... then figure out from there how many calories per serving.
If you don't have the recipe, you can try to find a similar one online. Here's a recipe for a heath bar coffee cake that uses buttermilk, maybe it's the one:
http://www.recipezaar.com/Heath-Bar-Coffee-Cake-74482 In this case, 1/15 of a 13x9" pan is about 300 calories according to that site... so the whole thing is 4500 calories... and to get 200 calorie pieces you'd need to cut it into 22.5 pieces -- that leaves you with
a piece that is just a little over 2x2". (13" x 9" = 117 square inches for the whole cake, 117 / 22 pieces = 5.3 square inches per piece, square root of 5.3 = 2.3" per side.)