If fertilizer has been added, then reliable companies will give the product's official NPK - and don't forget that most soil has *some* NPK without additives. (OTOH, if a company is constantly testing NPK as part of their manufacturing process, then it is unlikely you will get a bag of either very acidic or very akaline soil.)

You can do a little bit of sorting: both perlite and vermiculite improve the drainage capabilities. Vermiculite can also act as a water retainer while perlite is usually bulkier and helps lighten the soil (literally, it's very fluffy and lightweight). If the bag lists manure as an ingredient, be sure it says "composted manure", no way no how do you want uncomposted/raw manure. If the contents list has peat moss as the primary ingredient, most annuals will do okay, plants that prefer somewhat acidic soil will do well, and peat moss is a non-renewable substance so you have to justify it to your conscience. If the bag just says compost or composted soil, that's all you are getting, and you will likely want to add coarse organic material such as bark along with perlite or vermiculite to improve the draining capability before putting it in the pot.

"Soil" has a multitude of meanings... it's an organic substance; and apparently precisely which items contribute to the actual composition does not have to be specified on the contents label. It is possible to buy very good 'soil' aka 'sludge' (ummm, usually sludge is composted human waste) but check into the source since there have been instances of high metallic count in the finished product and you *don't* want that; you also don't want sludge that isn't certified to be disease-free.

In other words, unless you are a biochemist it is likely that no one other than the manufacturer knows exactly what is in that bag of potting soil. Use your nose: if it doesn't smell good, don't buy it. Potting soil is a convenience, not a magical formula.

Having said all that, I will say that I DO buy potting soil occasionally - a foot of snow over the compost pile tends to discourage me from digging even if it is seed-starting time. My personal guidelines (I don't have a favored manufacturer or brand) are: No bags listed only as "garden soil" because they are usually heavy, wet, with poor ingredients lists, and all too often smell bad - if I'm willing to deal with heavy and wet, I'll go out to my compost pile that has safe ingredients and doesn't smell bad. I prefer a bag that lists compost before peat moss, or preferably doesn't list PM at all (keeps my conscience soothed); I don't care whether it contains perlite or vermiculite because I keep both of those very inexpensive ingredients on hand and can easily add them. In point of fact, I sow all seed in vermiculite regardless of what is in the soil. I can't see paying a premium for a soil moisturizer added to the soil since those bags are usually several dollars more expensive for about a quarter's worth of moisturizer. I won't get it if sludge as one of the ingredients because I find it too aggravating to check out the source. Otherwise, if the soil looks good, feels good, smells good, and the price is right, I'll buy it.

Become a label reader. If there are additives to your commercial mix, it will say so. Stop by a locally owned garden center. They may be more likely to have ProMix, for example, than the big box stores.