• Look at the packaging and see if it seems counterfeit. Usually the lettres and images on counterfeit packaging are not as clear as on the the authentic one and the print is dull.
• Look at the brush head. If blue color of the bristles does not seem right, i.e. too faded, too turquoise etc. - chances are it is counterfeit.
• If the head takes some effort to get onto the toothbrush snugly, it is likely counterfeit.
• The Oral-B logo on some fake brush heads can be scratched off with your nail. Real Oral-B brush heads use high quality paint and it never comes off.
• After use, you may notice that your gums are red, and hurt.  This occurs because the bristles are not Braun Oral-B quality and they are hurting your gums.
• If the head breaks within a few weeks, i.e. head stops rotating - most likely it is fake. Original Oral-B heads  are of much higher quality, and they do not break easily.
• After a month of using the fake brush heads, the blue indicator may not fade, meaning that you are unaware that the brush has to be replaced. This is an indication that they are fake. Normally the authentic brush heads fade.

Do not buy Oral B Brush heads on eBay if the seller is from Asia, especially China, Hong Kong or Taiwan. Be sure, they are all counterfeit.