For a Will to be valid it has to be signed in accordance with legislation dating back to 1837. First and foremost, the Will must be signed by the testator. 

So, unless it is one of the rare situations where another person signs the Will on the testator’s behalf,  which must be in his presence and by his direction, the Will is invalid.

There are a wide number of forensic techniques available to detect forgeries which include handwriting analysis, electrostatic detection analysis, paper analysis, print analysis, ink analysis as well as witness statements.