The tape measure tool in SketchUp (keyboard shortcut T) is a very handy tool to use. It can be used to measure distances between two points or, the only reason I use it, to create guidelines for you to reference off of.
By activating the tape measure tool you are in the “guideline” mode where you can drop off a dotted line guideline along any line in your model. “Guideline” mode can also be verified by seeing a + sign next to your cursor. You can also click on a line you wish to reference off of and move the mouse in the direction you wish to drop the new guideline and either specify a distance and press enter on the keyboard or you can select another point on the model to drop it off. This will create a guideline that is exactly parallel with the original line.
To just measure the distance between two pints activate the tape measure tool and press the “Ctrl” button on the keyboard to remove the + sign. You will now be able to measure the distance between two points without adding a guideline.