A strong indication that a signature on a document may be a forgery is the lack of flow in the signature. When people sign their own names, they tend to do so quickly and smoothly.
Forgers who have to put conscious effort into signing a name not their own often leave behind subtle signs of their deception. Blots of ink from pen starts and stops, lack of line continuity from lifting the pen, constant line thickness from a slowly moving pen and shakiness in the lines are possible clues.
Some forgers do not try to imitate the legitimate signature's appearance. They simply sign the false name in their own handwriting. Others practice the legitimate signature before affixing it to the document in question, or they simply trace the signature.