Several antiquities dealers have either offered, or been offered, genuine ancient handles with suspicious-looking seal impressions (such as this one formerly in the private collection of Michael Welch).  Here are some tips to help spot them:

• Use a magnifying glass to make sure there is no evidence of bas-relief carving along the edges of the letters, icon, & border; this is both the easiest way to make a forgery, & the easiest way to detect one!
• Make sure its design matches one of the 21 types discovered in scientific archeological excavations; however, there's always the possibility that a rare type may be discovered by a chance find, not in a controlled dig.
• Make sure that the impression is of appropriate evenness based on the contour of the handle; it should not be even in appearance unless it is deep enough to be flat all the way across.
• Make sure its letters are smooth & narrow; however some genuine impressions may have letters that appear wide/flat if the ancient stamper squashed them during a secondary impression (see the Squashing section on the Tricks page).
• Make sure that its oval border is smooth & relatively even; however, some deformation can be expected on genuine specimens because the ancient stamper was usually not careful in the process (see the Sliding, Stabbing, & Shifting sections on the Tricks page).
• If the inscription AND the icon are mostly complete, be suspicious because nearly all genuine LMLK impressions are defective in one way or another.
• Make sure the ware color is consistent across all sections of the impression; however, some ancient impressions suffer from erosion that can expose inner sections of a different color; however, these spots should be randomly distributed rather than along design lines such as the letters & icon outline.

To further illustrate some of the points above, see the photos below.  They are grouped in pairs since some of the features are easier to recognize under alternate lighting.  The first 2 are of a forged impression followed by 2 genuine impressions for contrast.

The next 2 photos are of Redondo Beach #15, which contains a definitely genuine stamp, though there is no reliable method of proving that the Circles are ancient since they were incised after the jar was fired:

The next 2 photos are of Redondo Beach #53:

The next 2 photos were taken on oblique angles to highlight the differences between a genuine seal impression that was made in wet clay before being fired, vs. a carved impression made in fired clay.  The first one is Redondo Beach #62:

Notice the sharp smooth edges of the icon, letters, & oval border where they join the flat background.  Also notice the different heights of the letters, icon, & border.  In the forgery below, notice the carved outlines along those same features, & their relatively equal heights:

"Thus shall ye speak to Hezekiah, king of Judah, saying, 'Let not thy God in whom thou trustest deceive thee, saying Jerusalem shall not be given into the hand of the king of Assyria.'"--2Kings 19:10, Isaiah 37:10