The only true way to know that you are getting an genuine Sennheiser product is to purchase from an Sennheiser Authorized Dealer.
With great products and great success, unfortunately there are many "copycats" who try to pass off lower quality products as genuine - which they are not. Sennheiser works hand-in-hand with local authorities to take steps in combating counterfeiters.
Authorized Sennheiser dealers have a direct relationship with Sennheiser and uphold the highest standards for product integrity.
Purchasing Sennheiser products from an unauthorized dealer is a dicey gamble at best. There is no telling if these products may be stolen, returned, refurbished (even though they say they are new), new, old, or counterfeit.
Addititionally, if a product labeled "Sennheiser" is a counterfeit and stops working - there is no recourse. Warranty claims only apply to genuine Sennheiser products purchased from an authorized Sennheiser dealer.
If a price is "too good to be true", it generally is. Ensure you purchase only from an authorized Sennheiser dealer.