How do I know what type of radio I will need a license for?

Well, the frequency(ies) that it transmits on, and to some degree the power output, are the determining factors.  Whatcha got in mind?

Here's a band plan for amateur radio that shows what frequencies and power levels are allowed for various modes of operation and for different amateur license classes.

Most radios are advertised as being intended for specific radio services (i.e., the CB band, ham radio bands, Marine VHF service, etc.). Whether or not you'll need a license to use any particular radio depends on what services the radio is capable of transmitting on.

For example, radios that cover the GMRS frequencies (or "channels") will require you to have a GMRS license before you can TRANSMIT on those channels (although you don't need a license to RECEIVE on any channel in any service). The same thing applies for several other radio services:

If your radio is capable of transmitting on the ham radio frequencies, you'll need an amateur radio operator license that covers those frequencies, before you can legally transmit on them.

Ditto for Marine VHF radios: You'll need a Marine VHF license.

There are several services that don't require any license to transmit on:

- The channels in the CB (or "Citizen's") band.
- The channels in the FRS (or "Family Radio Service") band, and
- The channels in the MURS (or "Multi-Use Radio Service") band.

Note that many FRS radios also cover the GMRS channels - So, you could get by without a license only if you transmitted on the FRS channels, avoiding the GMRS channels.

Also, remember that LISTENING never requires a license, regardless of what kind of radio you have  - You only need the license to TRANSMIT.

Quoted: Ditto for Marine VHF radios: You'll need a Marine VHF license.

Actually, if you are using a VHF marine band radio on the water (and ONLY on the water) you do not need a license.  The license requirement went away in the 90's

wireless.fcc.gov/marine/fctsht14.html