Resolution is usually measured in arc seconds, which is an angular measurement of 1/3600 of a degree.

There are two ways to calculate resolution, the Rayleigh criterion, and the Dawes limit.

Rayleigh’s formula depends on the specific wavelength of the light, typically a yellowish green color at 550nm is used as a standard. While the Dawes method is not color dependent, both methods rely on the aperture (diameter) of the telescope.  Using the appropriate equation, in inches or millimeters, resolution results will always be in arc seconds.

Rayleigh resolution is calculated as 5.45 / Aperture ( in inches) or 138 / Aperture (in mm).  

Dawes resolution is calculated as 4.56 / Aperture ( in inches) or 116 / Aperture (in mm).

With the angular distance between two stars, such as two double stars, you can check whether your telescope could see that these are two distinctive stars, or whether it will blur the two into a single object. You can consult a star almanac for a list of objects with a resolution in arcseconds. Given good viewing conditions, you should be able to see anything with arcseconds higher than your telescope. Always remember this is a theoretical maximum based upon physics. Actual resolution results may depend on manufacturing quality.