Focus stop . This will prevent slide or lens breakage. If the scope doesn't have fine focus this is particularly important. You can tell a lot without test equipment. You're looking for an image that's really sharp from the center almost to the edge of the field of view, rather than one that must be refocused for each part of the circular field.
Achromatic lenses are highly desirable.
Uncorrected lenses will show color fringes around specimen detail. Apochromatic lenses, which are corrected for three wavelengths, are too expensive to consider here. The horizontal and vertical engraved lines should be straight. Rotate the eyepiece to check for the same problem with that lens. No internal dirt.