While even professionals find this a difficult task, you need to check on the quality of the food before you serve it to your bird, preferably, before you buy the food in the first place.
Food should not be wet, damp, mouldy or clumped together, dismiss any food like these immediately because it has not be stored properly during it's life time. One thing to look for that is easy to spot is vermin dropping, so move the food around to see if you can spot any.
Ensure to check the date of expiry on the foods labelling and where the foods originated. Smell the food as mouldy off food has a stuffy smell to it and if you really love your bird, put a couple of seeds in your mouth and taste them. Fresh seeds have a sweet taste to them.