From Onboard Updates:

"In response to FAA direction, United will no longer send advance notification to crew members who are subject to Drug/Alcohol testing upon arrival. The FAA  provided clear and unambiguous instructions, applicable to every U.S. carrier. They must comply with the no-prior-notification policy, or be subject to substantial fines. The prior-notification policy prohibits the use of ACARS, or any other means of advance notification.

While this is a procedural change for United, other U.S. carriers have never provided advanced notice to crew members being randomly tested. If you are subject to random substance testing, please remember that the people who notify you of your test are United employees, and are following strict procedures set by the FAA."