The question is "how will you prove". The act pf proving a document has nothing to do with Transfer of Property Act, but it relates to Indian Evidence Act.
Anything can be proved in hree ways- oral evidence, documentary evidence, expert evidence. In this case the receiver of gift will give his oral evidence as how gift was made, why gift was made, when the deed was execued, who all were present there, who were witnesses, did he get possession after deed, whether the deed was registered etc. He may bring other witnesses to state the relevant things.
Then he can prove by documents like letters etc. may have been written after or before execution referring to gift etc., other documents realted to his possession of property etc.
Expert evidence may be presented to give exper opinion on the signature of the executor, witnesses etc. Even if the witnesses has died, there signature may be available in various documents like bank, registrar etc.