How do I know which control cable I need?

In order to determine that, you need to know exactly which brand(s) of engine and control head are on the boat. 

Mercury/Mariner/Force and OMC/Johnson/Evinrude engines usually require an OEM-type cable when using those engine makers' control heads. OEM-type cables have special end fittings designed to connect to the control and engine with minimal hardware.

Generally, all other engines and controls use a "universal" (3300/33C-type) control cable. Universal cables have 10-32 threaded ends and often require extra hardware to connect to the engine and control. Contact the engine manufacturer if you need details on the hookup.

Diesel and twin station vessels may use a larger diameter (4300/43 or 6400/64-type) universal cables; 4300/43-type cables have 1/4-28 threaded ends and 6400/64-type cables have 5/16-24 threaded ends.