I mentioned the story of Usamah ibn Zaid and the man who pronounced the Shahaadah as a proof that we have to accept the Shahaadah of any one who says it, regardless of whether we think they are telling the truth or lying. We can then observe them AFTER this to see if they are telling the truth.
Narrated Usama bin Zaid bin Haritha: Allah’s Apostle sent us (to fight) against Al-Huraqa (one of the sub-tribes) of Juhaina. We reached those people in the morning and defeated them. A man from the Ansar and I chased one of their men and when we attacked him, he said, “None has the right to be worshipped but Allah.” The Ansari refrained from killing him but I stabbed him with my spear till I killed him. When we reached (Medina), this news reached the Prophet. He said to me, “O Usama! You killed him after he had said, ‘None has the right to be worshipped but Allah?”‘ I said, “O Allah’s Apostle! He said so in order to save himself.” The Prophet said, “You killed him after he had said, ‘None has the right to be worshipped but Allah.” The Prophet kept on repeating that statement till I wished I had not been a Muslim before that day. [Saheeh Al-Bukhari, Volume 9, Hadeeth No. 11]
It is narrated on the authority of Usama bibn Zaid that the Messenger of Allah (may peace be upon him) sent us in a raiding party. We raided Huraqat of Juhaina in the morning. I caught hold of a man and he said: There is none worthy of worship but Allah, I attacked him with a spear. It once occurred to me and I talked about it to the Apostle (may peace be upon him). The Messenger of Allah (may peace be upon him) said: Did he profess” There is none worthy of worship but Allah,” and even then you killed him? I said: Messenger of Allah, he made a profession of it out of the fear of the weapon. He (the Prophet) observed: Did you tear his heart in order to find out whether it had professed or not? And he went on repeating it to me till I wished I had embraced Islam that day. [Saheeh Muslim, Hadeeth No. 176]