Dear brother Muslim First,
Is there any advice from Hadith about praying behind or with someone who claims to be a Muslim but you feel that he does not quite come up to 'your' standard for a Muslim?
For sake of argument, what would you say to a Shia who refuses to pray behind Khalifa Muawiya or Khalifa Yazid?
What would you say to a neo-Salafi who refuses to pray behind a Shia?
What would you say to Malik Ishaq, head of the Lashkar-e-Jhangvi, recently released after a decade in jail. When a local journalist asked him what he now intended to do, Ishaq's replied that his organisation would continue its "good work" - fighting those who opposed their version of Islam. (Read 'murder the Shia apostates')
Suppose you follow Prophet and pray behind someone whom you consider to be pious. To be certain that the person is pious, you may follow someone who is nominated by a 'respected' authority to lead prayers. That authority could be the Caretakers of Haram in Makka or Chancellory of al-Azhar in Cairo or the Aga Khan or Sayedna. What do you think? Will you judge or let Allah be the judge and not use your intellect to discriminate?
[Bohras do not pray behind anybody, not even another Bohra, unless he has been nominated. If you do not know the person who claims to be Muslim, wouldn't you be safer praying by yourself so that you do not inadvertently negate Prophet's advice?]