There are many tools that exist to help evaluate the effectiveness of dairy cow's diet. However, the final judge to determine if a ration is performing well would be the cow herself.
Here are some things to look for that might indicate trouble:
Is the manure loose in general?
Are there any pieces of hay or haylage present?
Is there presence of occasional whole cottonseed?
Are there bits of corn kernels or whole kernels?
Ideally, rinsed manure should look "fuzzy" with fine particles which occasional bit of corn kernel or forage. However, if there are traces of ingredient that is big enough where you can identify which source they came from, that means there is opportunity to improve feed efficiency and milk production.
Sometimes, there will be some pieces of corn or alfalfa stem that gets through the system intact. When things like that happen, it does not mean something terrible is happening. If the milk production is good and feed efficiency is still up to par, observe more and look for any changes since the last time you walked the pens.
Manure evaluation should prompt some questions and point you towards the right direction. Loose manure is not the cause of low milk production. Instead, whatever that is causing loose manure is the real culprit that is causing low milk production.
There are be many causes for loose manure. If you are seeing corn bits, it may be because the grind-size or kernel processing is too coarse. If there are pieces of hay or haylage, is the neutral detergent fiber (NDF) digestibility low? Figuring out the various possibilities and arriving at the right conclusion with solution is crucial. Pay attention to what your cows are telling you and consult your vet or nutritionist if you need help.