It can be really hard to tell. About 2/3 of hollands are true dwarfs assuming the parents are both dwarfs, but there would be an even higher percentage of false dwarf if one parent was a false dwarf, so it’s a lot. If a rabbit gets one copy of the dwarfing gene it’s a dwarf, no copy and it’s a false dwarf, but with two copies it doesn’t survive, so any litter has a 1/3 chance of false dwarfs.
Say the dad is a false dwarf and weighs 4lbs – not that big at all and pretty much within standard. Mom is a true dwarf and very little at only 2.5lbs. Both rabbits are small by nature, whether or not they have the dwarf gene, so all of their babies will probably be small too and within standard for weight. They can still be bred to a true dwarf to get more true dwarf babies, so they aren’t necessarily sold as pets. I don’t know how possible it is to tell in very small babies, but it’s fairly easy to tell by 8 weeks what the rabbit’s adult structure will be.
My holland boy is likely a false dwarf, judging from his structure and 4lb size, but you would have to look very carefully and know about hollands to know that. Some are more obvious and much larger than standard, because their parents had genes that generally made them large and without the dwarfing gene they end up at 5lbs or so.
I hope this made some sense! Let me know if there’s any way I can clear up the explanation. I find the genetics of it very interesting.