When you have two in your hand, you can. Otherwise it is very difficult. veteran collectors can tell the difference by just weighing it in hand. Or you can rub it and if the pen smells rubber, it is made of HR. Or RHR is a bit more orange than the permanite Duofold.
BUT, you cannot tell from internet sellers such as ebay. I got mine from ebay because of the imprints. Duofolds with larger and medium imprints are almost surely made from RHR (there are exceptions, but those plastic duofolds with large and medium imprints are valuable too). I only learnt how to tell them apart by handling a couple of them.
When you have two in your hand, you can. Otherwise it is very difficult. veteran collectors can tell the difference by just weighing it in hand. Or you can rub it and if the pen smells rubber, it ...