By their tongue speakers' facial expressions, especially if they are looking around instead of even having their eyes closed or looking as they are focussed on God only. Also I went to 2 churches were it seemed all were saying the same like the leader. And similarly, in regards to special music and choir singers. I remember a special Easter presentation by a choir. Afterwards, I went to one of the performers who was a prayer warrior and told her that I noticed she had an angelic facial expression that no one else had. She gleamed and told me that she had prayed and fasted for 3 days that she would just be focussed on God and not on performing. I have complimented other worship team members, and they all have said that they are not performing but are just worshiping God.
And faking about interpretation of tongues is easily discerned by what is said not being Biblical and by differences of lengths.