People who get fooled are often only familiar with artists' names and not much else. You need to know what brush strokes look like, what favorite subject matters and compositions are, where signatures are typically located, and what mediums, materials, sizes and formats artists usually work in. Regarding examination techniques, black light has traditionally been used to detect irregularities on paintings, but recent advances by forgers are making this more and more difficult. Newly added signatures once flouresced and appeared to float above painted surfaces, but non-flourescing paints are now being used to counter this effect. Masking varnishes that impart overall translucent greenish looks to surfaces are also being used to hide inconsistencies.
Black light is still worth using, however.
It often shows previous restorations and gives other clues, positive as well as negative, to a painting's past. A good jeweler's lupe or 30-50 power pocket microscope can be used to identify locations where newly added paint has bled into nearby hairline age cracks. For example, if paint from a signature appears to run into the microscopic cracks directly beneath it, that could mean that the original surface paint dried, aged, and cracked long before the signature was added. Paper, either new or old, that has been glued over a painting's back. This is sometimes done to hide inconsistencies, condition problems, or manipulations.