The cardinal rule of purchasing a printer is to print before you pay. Otherwise, there's no surefire way to tell exactly how text and images will appear. Fortunately, many retail stores let you print demonstration pages to get a feel for the output quality.

Most demonstration pages will include rows of text at varying sizes, which can show different types of flaws. At the smallest font sizes, the individual letters should be legible and fully formed with no breaks, and they should not bleed into one another. Medium-size fonts should be crisp with no fuzzy edges. And the largest fonts, especially bold ones, should be filled in with a solid, even black--not a muddy bluish or brownish tone. If the tops and bottoms of characters are slightly offset or you see a pattern of dots incorrectly aligned from one row to the next (forming jagged outlines), that typically indicates misregistration of the printhead. You should also be able to see well-rounded counters (the openings) in letterforms; if not, that's usually a sign of the printer laying down too much ink. Keep in mind that on plain, 20-lb. paper, inkjet printers will usually display some wicking, as the ink bleeds along the paper fibers.
The printer demonstration should print several geometric shapes of different sizes and shading. The outlines should be crisp with smooth curves; inside areas of solid colors should appear dense and evenly shaded. Also look for areas where a color goes from dark to light (a gradient). Is it a smooth transition, or can you see color banding, distinct bands progressing from darker to lighter? Large areas of flat color should appear solid and even, rather than muddy. Some printers try to dazzle the eye with overly saturated colors; others skimp on ink, leaving images that appear washed out. Look for a nice, natural-looking balance between the two. Printhead banding--that is, visible horizontal stripes across a page--could be caused by a clogged nozzle, a poorly aligned bidirectional printhead, or a poor rendering algorithm (gradients aren't rendered smoothly).
When evaluating print quality, there are four chief considerations:
Color accuracy
Compared to the original, the colors should be accurate, pleasing, and well balanced. Colors should be vivid but not oversaturated. Look at a monochrome photo under fluorescent light, incandescent light, and daylight. How badly does the color cast change from one light to another (called metamerism)? Be sure to look for inconsistencies across different paper types and print resolutions.

Is the output sufficiently sharp? Any jaggies? If you see problems, do you have any theories about what's going on? Does the lack of sharpness have to do with printhead or color misregistration?

Dynamic range
Can you see detail in highlight and shadow areas, or are they rendered as flat or with no color? Does the printout look muddy or low contrast?

Do you see any banding? How about evidence of a clogged nozzle? Excessive dot gain? Any other weird stuff going on?