Cartier glasses serial number 6138821. Are they real?

It may be possible to tell from a serial number that they are definitely fake (because no real ones with that serial number exist). However, it is not possible to definitively state that they're real; putting a real serial number on fake glasses is considerably easier than making the fake glasses accurately enough that they seem like they MIGHT be real in the first place. If you want to know for sure, take it to an appraiser; we can't tell you from the serial number alone that they're real.