If by "observe" you mean opening up and looking, yes. Plenty of visual examination, too. How do they behave, how fast do they grow, how big do they get, how productive are they (eggs/hen), what's your hatch rate, how disease resistant are they? When you take them apart and look inside (as I am doing this weekend - freezer camp!), how are the internal organs? Oversized kidney, liver are signs of issues. Fat levels. Even the condition of a fresh cracked egg. How "rich" does it taste, how quickly does the albmin set, does the yolk stand up firm and high, or is it sort of shallow and spread? Yolk color is deceptive - it can indicate high levels of some vitamins - like beta-carotenoids, which can be converted to Vit A - but it can also be from less nutrtionally valuable pigment sources. I consider it mostly "marketing".It's because you have to obverse your flock, right?
Say you grow a clover and white millet cover crop (as an example), and they mostly eat... the clover. The following pasture you have to remove the millet and replace it with another plant they might like, and keep the clover and maybe increase the amount.
You have to do trial and error to figure out what they like to eat.
Even then, you don't have certain knowledge, because you have no basis for comparison to determine if your birds could be doing better, if only they had more of something.
That's where people tend to make the most mistakes. "My birds look fine" compared to one another may be true, but it has little to do with how much better they might be doing.
Ultimately its risk management, and risk/reward. I feed my birds as well as I can within a price range I find acceptable. Increases in feed costs to obtain a superior feed are significant - I'm at one of those plateaus where a little improvement would cost a lot - and the research says the benefits of those improvements (at least gross improvements like crude protein) are expected to be very small. So instead of buying more expensive feed, I put $100 to %150 a year (one month's feed bill) into improving my pasture to add biodiversity and improve soil condition.
Oh, and knowing what I do about feed, and my management methods, I take some feed cost savings methods which I KNOW have long term health consequences for some of my birds - I simply plan to cull before those consequences are measurable. I consider it educated risk at acceptable levels. Should I find myself mistaken, in the fullness of time, I'll change methods, or feeds, or both. But I don't recommend people do as I do. I recommend they do their research and decide for themselves.