Foam used in upholstery is called polyurethane (PU) foam. Its quality is mostly determined its density. The density is the weight of the foam per cubic meter.
Do not confuse hardness/firmness with density!
Although low density foam is generally very soft, and high-density foam harder, this isn't a rule. Fillers are used to make hard low-density-foam to give the impression of a higher density.
And most high-density-foam product used in upholstery are actually relatively soft to ensure the necessary comfort level.
Too many retailers claim all over the internet using “high density foam” In most cases, those are false claims. Most couch manufacturers actually use a medium medium density foam. Depending on the quality, this type of foam can handle regular users of up to 90kg.
Few unscrupulous manufacturers uses low density foam or flocks
Not all foam, including of the same density, are created equal. All foam manufacturers use "fillers" in more or less quantities. Calcium Carbonate for instance adds volume, hardness, and weight, but has no resilience. Using too much filler will result in the foam's collapse.
It is possible to source "pure foam" with no added filler.
The best way to be sure is to feel the weight of the cushion and unzip the cover to see for yourself what was actually used.
But here is the trick; most manufacturers who use lower density foam, manufacture their product with fixed seat (built into the frame). It is also a way for them to use less fabric...
Despite the fact that you won’t be able to see what type of foam was used, the main concern here is that if the foam collapses and it will eventually anyway, you will not be able to replace it yourself. You will have to take it to a re-upholsterer and the cost involved will be higher than purchasing a new couch.
The other advantage of a loose seat cushions is that you can turn it around frequently to increase the life span of the foam. I would therefore recommend if you want to keep your couch for a long period of time, that you buy one with loose seat cushion.
Remember that most manufacturer excludes “fabric and filling” in their warranty. It is therefore very important, if you or someone in your family who will be using the couch ways 90kg or more, that you disclose that information to the seller. You should rather pay a little extra for a higher density foam to be used at once than having to replace it in the next 6 months.