The TIA-606-B standard for labeling and administration of structured cabling systems - recently approved by the TIA as well as ANSI - includes a standardized symbol to identify ports that carry Power-over-Ethernet direct current.
In an article that will appear in the April issue of Cabling Installation & Maintenance magazine, Hellermann Tyton's Todd Fries, a member of the TR-42.6 subcommittee that built the 606-B standard, explains, "Certain applications may provide electrical power in addition to data transmission over balanced twisted pair cables. Visual identification of ports with power may be accomplished through the use of this symbol." The symbol, which Fries adds can be included anywhere on a label, appears at the bottom of this page.
The emphasis on the word "may" indicates the use of the symbol is permitted but not required by the standard.