Is it permissible to pause and ascertain the true nature of individuals whose circumstances are unknown, especially in Egypt where polytheism (shirk) in its various forms has become widespread?

The assertion of requiring full discernment (tabayyun) is an innovation (bid'ah). The fundamental principle is to rule individuals in a Muslim society as Muslims unless an invalidator (naqid) clearly emanates from them. One must have a good opinion of Muslims, and having a bad opinion of them is forbidden, as Allah the Almighty says: {O YOU WHO HAVE BELIEVED, AVOID MUCH [negative] ASSUMPTION. INDEED SOME ASSUMPTION IS SINFUL}.