One common question that is asked of Techs at Digi-Key is to trying to verify the correct replacement wall mount AC to DC power supply. The difficult specification to find on many of these power supplies is what size if the inner diameter of the plug / connector. This is made more difficult by the fact that the two most common sizes of plugs at 2.1 mm ID x 5.5 mm OD and 2.5 mm ID x 5.5 mm OD. Since this measurement is too close to use a ruler and fitting a caliper (if available) inside a 2.1 or 2.5 mm hole is very difficult to do accurately. This leaves you with two options. You can buy one and get an Adapter (2.1 to 5.5, or 2.5 to 2.1) in case you buy the wrong one, or there are a couple tricks that can help identify which size you have. Please make sure the power supply is de-energized before trying any of these methods.
Most round toothpicks are manufactured to be between 2.1 and 2.2 mm in diameter. If you slide this into the end of a 2.1 mm ID plug it will be snug and would not fall out when tipped up. When placed in a 2.5 mm ID plug the toothpick is loose and will fall out when tipped.
If you take a pen the widest point on the tip is going to be a standard 2.25 mm - 2.35mm. This means that the entire tip will not go into a 2.1 mm plug, however will go into a 2.5 mm plug.
This is the a little bit more of a trick. Most test leads you will find have a diameter of 2 mm and fit in both the 2.1 and 2.5 mm plugs so you have to look at the gap. Being very close to the same on the 2.1 you will notice little to gap between the test lead and the barrel plug. When placing the test lead in the 2.5 mm plug you will see a noticeable gap around the lead.