If they start to look thin, stop laying eggs, are getting sick, etc, then you could be underfeeding them if they have no food left in their feeders when they go to roost. I'd just take the trial and error to get to the point of having just a small bit of uneaten feed in their feeders when they go to roost. Then you'll know they are getting enough. The only problem with this method is some birds are just less thrifty with feed...they eat more than they need and therefore poop a lot more. But I wouldn't worry about that too much, I'd focus on making sure they're getting as much as they want. You will for sure notice a big drop of feed waste.
Now if they are truly wasting feed, tossing it all over the place, letting it pile up on the ground, that isn't good. You don't want that for the obvious waste reason and it will attract rodents. If they are wasting feed like that, then I would reduce their feed until they learn not to waste it. Let them go to bed hungry for a few nights...I don't mean to not feed them anything, but only feed them as much as they will eat without wasting anything. Again, this will take some effort on your part, noticing what they are doing and adjusting accordingly, but it will save you a lot of money and headache down the road. Also, making or buying a better feeder that helps reduce waste, like the homemade one someone here suggested. There's a big thread about it on here.
We use a treadle feeder that helps reduce both waste and rodent issues. We made ours. There's the Grandpa one which I think is around $300 or there is the all-metal one, a bit more flimsy but still functional for about $60+shipping.