The third criteria according to the Holy Books to know the truth of a true Prophet is his speech, is it holy or not? If a person is claiming to be a Prophet and he says that there is no God or you do not have to follow God’s commandments etc, then we can know from his speech that he is not Godly. A true prophet of Allah will explain about Allah, His commandments and guide his people to the right path.
The Holy Qur'an relates:
And We send not the Messengers but as bearers of glad tidings and as Warners. So those who believe and reform themselves, on them shall come no fear nor shall they grieve.
The Holy Bible relates:
Jesus of Nazareth which was a Prophet mighty in deed and word before God and all the people.
Holy Bible - Luke Chapter 24, verse 19
Concerning Hadhrat Mirza Ghulam Ahmad, may peace be upon him, he wrote over 80 books, so anyone can read any of these books to see what he has said. Do his words bring you closer to God or move you away from God?
Here is a link to one of his books called 'Philosophy of the teachings of Islam' http://www.alislam.org/library/books/Philosophy-of-Teachings-of-Islam.pdf
Now there are a lot of accusations about what Hadhrat Mirza Ghulam Ahmad, may peace be upon him, has written or said. We will cover some of these in the next stage. But many of these have been misquoted, taken out of context or just lies. Here is a link to all the books of the Promised Messiah in Urdu or Arabic, as they were first written: http://www.alislam.org/urdu/rk/
If anyone claims that Hadhrat Mirza Ghulam Ahmad, may peace be upon him, wrote something which seems unholy, so now you can look up the original quote and see if it was actually written or not and by looking above and below the quote, you can get the real message that he was explaining.