How can we recognise a great person who has attained God from the exterior? Would we be able to identify such a spiritually elevated individual from their face, speech or conduct?

One can recognise a great person who has experienced God through his conduct.  This is why Arjun asks God about the characteristics of a person with steadfast wisdom in Gitaji.  How does a person with steadfast wisdom speak, sit and talk? Also, in Shri Ramcharit Manas, Lord Ram Himself explains the qualities of such saints to Naradji in Aranya Kānd and in Shrimad Bhagavat, Lord Kapil explains these qualities to Maa Devhuti.

Aspirants wanting to know this should study these sections in these scriptures. There is no doubt that once we come into contact with such divine people or saints, then experiencing God becomes simple and successful in their association and guidance.