Wifi - Determining the channel of a probe packet
The Beacon frame itself contains this information. 

The 802.11 spec describes this as an optionally-present field on a beacon frame denoting a channel switch; however beacon frames may carry this for a basic beacon packet not denoting a channel switch.

Probe responses also carry channel usage info if the request specifies to send this.

In addition, certain monitoring or capture software could potentially read field 7 of association request packets, which describes the supported set of channels by a client