How does one know if one's US Passport has been revoked while traveling abroad?
22 CFR § 51.65 Notification of denial or revocation of passport. (a) The Department will notify in writing any person whose application for issuance of a passport has been denied, or whose passport has been revoked. The notification will set forth the specific reasons for the denial or revocation, and, if applicable, the procedures for review available under 22 CFR 51.70 through 51.74.
I imagine in practice the first time a traveler would become aware of the revocation, if abroad, and if nobody at home opened the mail, would be when they were (at best) denied entry or exit by authorities that share information with the US.