Two criteria to judge the soundness of the person's religion:

بـسـم الله والحـمـد لله والـصلاة والـسـلام عــلى رسـول الله، وبـعـد

a. By Their Outward Actions:

Narrated Anas bin Maalik: Allaah's Apostle صلى الله عليه وسلم said, “I have been ordered to fight the people till they say: 'None has the right to be worshipped but Allaah.' And if they say so, pray like our prayers, face our Qiblah and slaughter as we slaughter, then their blood and property will be sacred to us and we will not interfere with them except legally and their reckoning will be with Allaah.” Narrated Maimun ibn Siyah that he asked Anas bin Maalik, “O Abu Hamza (kunya of Anas)! What makes the life and property of a person sacred?” He replied, “Whoever says, 'None has the right to be worshipped but Allaah', faces our Qiblah during the prayers, prays like us and eats our slaughtered animal, then he is a Muslim, and has got the same rights and obligations as other Muslims have.” [Saheeh al-Bukhaaree]

Narrated `Umar bin Al Khattaab: People were (sometimes) judged by the revealing of a Divine Inspiration during the lifetime of Allaah’s Messenger صلى الله عليه وسلم but now there is no longer any more (new revelation). Now we judge you by the deeds you practice publicly, so we will trust and favor the one who does good deeds in front of us, and we will not call him to account about what he is really doing in secret, for Allaah will judge him for that; but we will not trust or believe the one who presents to us with an evil deed even if he claims that his intentions were good. [Saheeh al-Bukhaaree]

b. By Their friends:

Narrated Abu Hurairah: The Prophet صلى الله عليه وسلم said: “A man follows the religion of his friend; so each one should consider whom he makes his friend.” [Sunan Abu Dawood (4833) and Tirmidhee (2378), and graded as “Hasan” (good) by Shaikh al-Albaanee]

Abu Qilaabah said, “May Allaah fight the poet [an expression of amazement in `Arabic] who said: Do not ask about a man, but ask about his companion. For every person guides himself by his companion. This poetry is by Adee bin Zaid, and al-Asma’ee said about it, “I have never seen a line of poetry which resembles the Sunnah more than this saying of Adee bin Zaid.” [Al-Ibaanah (2/439)]

Mu’aadh bin Mu’aadh said to Yahyaa bin Sa’eed, “O Abu Sa’eed! A person may hide his viewpoint from us, but he will not be able to hide that in his son, or his friend or in the one whom he sits with.” [Al-Ibaanah (2/437)]

Imaam Al-Awzaa’ee said, “Whoever hides his innovation from us will not be able to hide his companionship from us.” [Al-Ibaanah (2/476)]