As usual, A. T. Hagin nailed it pretty good.
I look for it being on sale. I know it sounds funny, but normally, pellets are just too pricey for me. But, we have a regional feed maker called Armada. They are a small company with really great products. I really like their pellets. Twice a year, a small chain of rural stores, who sell the Armada Feeds, puts the feeds on sale. $2 a bag off. At that price point, pellets become something I buy. I can feed them for convenience, toss them like scratch, etc.
The other advantage of having a sale is that the stores generally bring in a fresh pallet of the feed and set it in the aisle. It is only days old. Great. That means I can keep it for a month or two or three in dry storage, and await the next sale. If it were already 3 months old, I'd steer clear. Moldy clumps? Dump it. The risk is too high.