My trick is to look carefully at the food I'm about to eat, and consider how good/filling/satisfying/etc it looks compared to a known benchmark (e.g., a McDonalds cheeseburger). Estimating that way, I'm usually within 20%, which is plenty close (esp given that unbiased errors cancel out).

Beyond that, weigh every day, track values in a decaying average app to cancel out noise, and greatly reduce or fast completely on days where I'm over my line.

Easier said than done, of course, but I'm halfway successful.