Take a look at this closing of a letter from Martin Luther King, Jr. King did sign a lot of his letters, but does anything look "off" about this particular signature?
This signature is a stamp. What gives it away? First, many stamps during this period are blue. So it was with Einstein's stamp, which he used to sign many of his advocacy letters. But this alone is not enough. Some pens are blue.
Look at the way the ink spreads irregularly and there are skips toward the end of King. These are other signs that a signature might be a stamp.