First of all manure is not compost. Animal dung is an ingredient that goes into compost. Give it six months or more of composting and it might be compost.

Eliminate all the brands that have sand in them. Sand is common filler used to raise the weight. Put a handful of compost in a quart jar and swirl it around. Sand sinks. You can usually see sand at the store just by holding it in your hand in the sunlight. Sand particles sparkle like salt. If you buy compost with sand, you should get a 2/3 discount over better materials.

Smell the compost. If it is finished composting, it should smell fresh like a forest floor after a summer rain. If it smells sour, dank, moldy, yeasty, musty, or otherwise rank, it is not finished composting. Or it might have been bagged wet or sat out in the rain. That fresh smell comes from certain microbes. It takes a long time of composting to get a high enough population of these microbes to smell fresh. Your nose is the best tool for identifying finished compost.

Excellent compost will have been screened to filter out the large pieces of anything. If you recognize anything in the bag as being formerly alive (like twigs or pieces of bark), it was not screened very well. That stuff will need to compost some more and will "rob" nitrogen from the rest of the compost.

I have looked at bags of Scott's, LETCO, Earth's Finest (also a LETCO material if you read the fine print), and Black Kow. Earth's Finest and Black Kow were the only ones that passed my tests above. The Black Kow had a lot of woody pieces but so did the rest. Scott's and LETCO were very sandy.

They don't add sand to increase weight. For commercial compost producers, weight is the enemy. They get paid based on volume. The more a cubic foot or yard of compost weighs, the more it costs to ship. That eats into their profit.

Also, if you buy from a reputable company and there are wood chunks in it, they won't be a nitrogen draw if the material has been nitrogen stabilized. It will tell you right on the bag or the paper work if they have done that or not (Again,from a reputable company). Sure, it's not completely composted, but it will break down in the soil over time without robbing nitrogen.

Locally made compost is the best... minimize shipping, reuse of local materials. We have Better Earth compost made locally here.

Our city compost site sells bagged or bulk compost called "Garden Green". If your area has a city site, they undoubtedly have a commercial sideline too. Even though I make my own compost, which is good stuff, I bought a few bags of the Garden Green for a comparison and couldn't find anything wrong with it at all.

They don't add sand to increase weight. For commercial compost producers, weight is the enemy. They get paid based on volume. The more a cubic foot or yard of compost weighs, the more it costs to ship. That eats into their profit.

Uh, good point. I was confusing 'sold by weight' products with 'sold by volume.' I wonder why I see so much sand in bagged compost??? In my pitiful defense, I hardly ever buy bags of stuff.

Also, if you buy from a reputable company and there are wood chunks in it, they won't be a nitrogen draw if the material has been nitrogen stabilized.

What does 'nitrogen stabilized" mean?

Another suggestion for the OP is to find a horse stable to supplement your compost. Sometimes you can find a stable that has been piling manure for years and it is already composted. And of course there is other people's leaves. And you can look for tree trimmers chipping branches into a truck. That stuff takes a little more time to compost but the mix of protein to carbohydrates is just about perfect with all the leaves and fresh woody parts.

You see varying amounts of sand (along with silt and clay) for a number of reasons. A lot of the time it gets unintentionally included in the mix as they're turning it and moving the piles around on the ground with skip loaders. Another source of dirt is manure. Cows in particular eat a lot of dirt as they graze and that's just going to be a component of their waste.
Nitrogen stabalized just means that they've added enough nitrogen to balance the carbon. The compost is analyzed (hopefully) at some point near the beginning of the process to determine the nitrogen percent (both immediately available and total) and a C:N ratio. Then you can calculate about how much nitrogen you'll need to add near the front end of the process (usually urea) to keep the C:N ratio reasonable. If they're smart, they'll test it again before shipping it out and adjust as necessary.

In their advertising hype this company states they produce the only true (as defined by the USDA) compost. That leaves us with a dilema since the last time I looked the USDA had several definitions of what compost was.
If you must purchase compost know that this is a buyer bewary item to purchase and the best thing to do is perhaps buy one bag and take a very close look at what is in that bag before determining whether to purvchase more. Since nothing regulates what is supposed to be in a bag of "compost" it can be anything, except possibly sewage but then again.