Star Test
The acid test for any telescope is the Star Test. which is carried out by studying the Airy disk produced by a bright star. If you live in the Northern Hemisphere, Polaris is your best choice. While it is a faint star to the naked eye, it is a bright star to your telescope. Polaris is preferred to other bright stars because it remains relatively fixed in position due to its proximity to the Earth's Polar Axis.

Before conducting this test, your telescope must be allowed to acclimatize to the night's conditions. Pick a night with good seeing conditions for this test. For telescopes in the range from 6" to 10",- mutltiply the number of inches in your telescope's aperture by 25.  This will give you the desired magnification.  My 8" telescope should be set for a Magnification of 200X.  The correct focal length of the eyepiece to use to get this magnification is your telescope's focal length in millimeters, divided by the Magnification just calculated.  For my telescope this would be: 1371.6 / 200 = 6.858 or 7mm.  Note this yields an exit pupil that is very close to 1mm. Thus if you know your telescope's Focal Ratio (F: number), simply use an eyepiece with a focal length equal to your telescope's F: number. My Telescope has a Focal Ratio of 6.75 so again I would use a 7mm eyepiece.

The test is conducted by getting the best focus of the star that you can. Then VERY SLIGHTLY defocus the star on both sides of the best focus, and compare the resulting Airy disks. Any lens or mirror aberation(s) afflicting your telescope will distort the ideal Airy disk in a characteristic way, which is often different on either side of your best focus.