When we pray, after all set and done, how would one knows that prayer is being answered immediately?

If your spiritual father/mother is at present he/she would tell, that the prayer is answered.

But in situation where you are alone say in office or house, how would one know instantly that a prayer has been granted? would it be true sign, certain feeling etc

I remember a Pastor Uche in this section has written article that is similar that answered this question in different heading.
I think when prayer is being granted or reaching God you will feel joy inside you, during and after and eager to pray longer during the session.

I think Rev.Fathers and Pastors in the forum are more in better position to answer this, being their field.
You see, the Christianity we practice these days (new testament) is different from that of old testament where instant answer is given. Eg fire consuming sacrifices.
So this time around you have to use your head or faith to accept that God has granted your request(s).