Radio frequency devices that are imported into the Customs Territory of the United States need to be declared with CBP / FCC via form FCC 740. PDF version of the form is made available by FCC here, while MS Word version of the form can be downloaded here.
Many frequency devices that are introduced to the commerce of the United States are required to be licensed. The licensed transmitters will have FCC ID issued to them by the Federal Communications Commission. When filling out the FCC740, you are asked to provide the FCC ID. FCC id can be verified through the agency’s website utilizing Equipment Authorization Search.
Some frequency devices may not require license. See FCC’s Office of Engineering and Technology Article.