The Morgan on the left is cleaned; the right has slide marks.
Harshly cleaned coins are common in Proxibid and eBay auctions. This 1884 Morgan dollar (above photo, left) shows deep hairlines from cleaning with a metal pad, essentially ruining the coin. You can see the arc of the cleaning here, curving downward and across the cheek.
(Click the photo to expand.)
Other kinds of cleaning are more difficult to detect.
Lightly cleaned coins still have hairlines from rubbing with a cloth rather than metal pad. Again, these hairlines are seldom straight as in a horizontal or vertical streak; they usually are uneven or curved, as in a rubbing pattern.
Horizontal or vertical streaks often are so-called "slide marks" caused by coin albums with plastic slip protectors. Dansco albums, for instance, have such protectors. When hobbyists are not careful and "slide" the protectors to take out coins, the dust on the plastic rubs against the metal and causes those straight lines.
Slide marks may lower eye appeal, but coins with them still are grade-worthy. Harshly and lightly cleaned coins will not earn a grade from the top holdering companies (PCGS, NGC, ANACS, ICG).
The most difficult cleaned coin to detect in online photos comes from dipping in a solution that strips a thin layer from the metal. NGC has an informative article about dipping.
Many dealers are skilled in dipping to remove tarnish, but some hobbyists who try this--using a solution like Jewel Luster--strip away too much of the top layer. The coin ends up losing the luster (ironically), leaving a dull surface.
The problem with online photos of such coins is the flash of a camera often bestows a "false" luster (light shining on metal) that the coin actually lacks.
To avoid spending top dollar for cleaned coins, consider ones only in top-tier holders Or ask the seller whether the coin has been dipped or otherwise cleaned. Never bid on a coin with poor photos that prevent you from discerning true condition--no matter what the seller says about the lot or what the flip states about mint state or value.