We use a cup to measure the dogs' food - we know that two-thirds of the cup is the right amount for them, normally - and I top it up with at least a cup full of water, per the Dogs' Trust's advice when we got ddog2.
Usually ddog2 is on normal dog food - she is, we think, a lab-pointer cross (she's a rescue dog, and the Dogs' Trust said lab cross - people with pointers see pointer in her), and she's normally a very active dog indeed - happier running than walking.
Sadly, last November, she ruptured both cruciate ligaments, and had two lots of knee surgery in spring, to mend those - necessitating 15 weeks cage rest - and was just getting back to a normal level of activity, muscle power and stamina this autumn, when she dislocated her knee cap, and needed a third orthopaedic op (to move the bony prominence the muscle holding the kneecap in place attaches to) - and that meant straight back onto cage rest, 11 weeks ago. Supposedly for 6 weeks, but the bone hadn't healed as well as they'd like, so they said another 4 weeks - then last Friday, on examination, her kneecap was more mobile than it should be, so she is still on cage rest, and will be until her next lot of x-rays and appointment - on January 8th!
Oh - and just to add to her woes, the anti-inflammatory they put her on, when she dislocated the knee, gave her a stomach ulcer!
As a result of all this, we have had to cut down on her food intake, as she is having very little exercise at all - so she is on the light food (so she doesn't feel hungry), and a bit less of it, too.
I tend to judge it by eye - both dogs look healthy - both have nice waistlines, though ddog1 is a more solid looking dog, even at her most ideal weight - big head, broad shoulders, deep chest - so she is never going to have ddog2's supermodel figure!
Mine both like raw veg and fruit - I give them the ends of carrots, when I am chopping them up, or the end off the cucumber, cabbage leaves, or the outer sticks of celery, apple cores - stuff like that (though less of the fruit, because of the sugar - that is very occasional) - they think they are getting a treat, and I know it has little in the way of calories, so it's a win-win.