On a sectional chart, airport information such as a CTAF frequency is shown in the airport data block. The CTAF is identified by:
AA solid magenta dot next to the frequency (or a 'C' inside a filled circle in some chart formats)✓
BA blue square around the frequency number
CA letter 'T' next to the frequency
Why →The Common Traffic Advisory Frequency (CTAF) at non-towered airports is shown in the airport data block, typically marked with a solid magenta dot (or 'C' inside a filled circle, depending on chart series). The CTAF is used by pilots to announce position and intentions at uncontrolled airports.
Field note →You do not need a radio under Part 107, but near a non-towered airport a $100 handheld aviation radio lets you listen to CTAF and know who is in the pattern before you launch.
SOURCE → FAA Aeronautical Chart User's Guide; AIM 4-1-9CHECKED JUL 16ACS II.A.K5EASY