DC (Direct Current) is the alternative to AC (Alternating Current) in modern electronics. DC is usually used with lower-voltage devices that have other power supplies, such as batteries. For example, you can plug a DC power cable into a device to charge a battery and the current will only be going one way -- to the battery. There are a different types of plugs used in DC power, but there are quick ways to tell them apart.
Place the end of the measurement tool against the base of the DC power plug. Make sure the millimetre side of the tool is flush against the shaft of the plug.
Locate where the end of the plug is on the measurement tool.
Read the display on the tool. You are likely going to have either a 10mm (standard) or 14mm (long) plug. There are other sizes for various kinds of electronics, especially computers, but these are the two most common sizes of plugs.
Look in the "Technical Specifications" section of your device manual and you will find the type and size of DC jack listed.