The DEA number on the forged prescription is incorrectly formatted. All DEA license numbers have nine digits: two letters and seven numbers. The first letter indicates the type of prescriber (Hospitals are assigned the letter B, individual practitioners are assigned the letter C, mid-level practitioners (Nurse Practitioners, Physician Assistants) are assigned the letter M). The second letter is the first letter of the prescriber's last name, so Watson's DEA number would start with the letters BW.
Watson goes through a list of phone numbers Marla called and one stands out as "weird" to him - "929 555 0196" in "Queens, NY", circling it - recognized as her having called it two weeks prior. More weird is not only did every non-blocked number have a 555 prefix, typical of television and usually not worthy of note, but was also always followed by 01. Holmes recognizes the number by its "prefix" as being a private exchange. Again, every prefix was the standard fake prefix "555", so not as insightful of an observation worthy of Sherlock Holmes to make.