How to know which radio frequency is free to transmit data over a particular frequency to avoid interference
In general terms, equipment on any band needs to be licensed for use there.
Some bands are licence-free, such as the 433MHz band in Europe, the 902MHz band in USA or the 2.4GHz band world-wide.
In order to use a license-free band, the equipment still needs to conform to various regulations. In Europe this is the Radio Equipment Directive + harmonized standards, in USA it is the FCC Part 15 Directive.
The general policy when two conforming devices make access to a license-free band is first come, first serve. Specific bands will have specific requirements such as narrowband channels, &quot;low duty cycle&quot; transmitters (only send 10% of the time etc) or pseudo-random frequency hopping (as seen in FCC Part 15).
Some bands and technologies implement &quot;clear channel assessment&quot; which either means that they simply read RSSI and sit and wait until the channel is free, or implement some smarter algorithm for keeping track of the band and finding a free channel.

Peeking at your profile, I'm guessing this is for Nigeria? I can't find anything about license-free bands in the frequency allocation table from www.ncc.gov.ng, other than 2.4GHz. They use exactly the same technical EN/ETSI standards as EU though.