This sounds bad, but it how you can figure it out.
The plastic most toys are made with is very cheap and not very durable. It may look and feel like quality, but the actual chemical make up is very different from the synthetic materials used to create the quality and strudy real ones.
What I found out.
All plastic, will be eaten/melted by paint thinner or turpentine. This is because turpentine and paint thinner are designed to break down the chemical makeup of paint, which happens to be very similar to plastic.
Cheap, PVC toy plastic reacts quickly to paint thinner. The quality synthetics, do not react as quickly because they are not true/pure plastic. They are a little different, this is what makes them so strong. Buy some paint stripping paste from a hardware store. Find a small spot on your stock that wont be a big deal if it gets messed up.
If the stock is a cheap airsoft knockoff, the paint stripper paste will literally melt the plastic almost instantly. You need to leave it on for a few minutes to really see. The hard/better plastics will not react as quickly.
Case in point. I had a cheap DPMS 4 position stock that was painted. I also had the pistol grip painted as well, but it was made of a much harder synthetic. Well I put the paint stripper on both parts (i knew it had the potential to melt plastic, but it had been my experiance before that with good plastic it takes a while, usually after the paint has already been stripped). Well the DPMS stock instanlty began to melt. You could take your fingernail and scrape the plastic away. However the pistol grip did nothing, it was fine. The same with the bushmaster handguards, they were a little in between the pistol grip and the stock.
So maybe this is a dumb idea, but its the only way I know of to determine what kind of plastic anything is made of.