When performing protocol analysis, you would look at the HT Information Element in the Beacon frame to determine which one of the four HT protection modes the AP is operating in. The HT Information Element contains various subfields that provide information about the HT network configuration and operation. One of these subfields is the HT Protection field, which indicates whether any protection mechanisms are required for mixed-mode operation with non-HT STAs.
The four possible values for this field are:
No Protection: No protection mechanisms are required.
Non-member Protection: RTS/CTS or CTS-to-self protection is required for all HT transmissions.
20 MHz Protection: RTS/CTS or CTS-to-self protection is required for all HT transmissions using a 40 MHz channel.
Non-HT Mixed Mode: All HT transmissions must use a non-HT preamble and header.