If you are going outside of legitimate or recognized or trusted sources, then yes, they could definitely be counterfeit. Maxim-Dallas has a list of valid distributors here: http://www.maximintegrated.com/sales/offices/distributor/franchise.mvp#
Mainly Digikey, Mouser and Avnet in the US. Outside of these chains, buyer beware.
You could be buying one of multiple parts. Best case scenario, Parts Pin and Code compatible. Then you have parts that are from Ghost Shifts, where legitimate manufacturer plants/equipment is used off the clock to produce extra parts. You could have parts where legitimate runs are deemed defective (say 10% of a run's samples are bad, they scrap the entire run) but instead of trashed someone steals them for resale. You could have unauthorized parts designed to meet the specs but under quality. Or the worst case, you have duds that are relabled or worse, parts with no electronics inside (I.E. Just Epoxy and Leads, no wafer).
The only way to tell if the parts are good is testing EVERY SINGLE ONE AND PRAY. You could order samples (free or paid) or a small quantity (say 10~20) but you might end up with good parts (or parts that will take extended testing to break) intended to trick the buyer (you) into ordering a larger quantity, which will end up being lemons.
Basically, Cavet Emptor.