The evidence is his persistent employment of the second person plural.  For example:

“The God of peace be with y’all.”  (Romans 15:33)

“Now I want y’all to speak in tongues, but even more to prophesy .... I thank God that I speak in tongues more than y’all; nevertheless, in church I would rather speak five words with my mind, in order to instruct others, than ten thousand words in a tongue.”  (1 Corinthians 14:5, 18)

“My love be with y’all in Christ Jesus.  Amen.”  (1 Corinthians 16:24)

“And I wrote as I did, so that when I came I might not suffer pain from those who should have made me rejoice, for I felt sure of all of you, that my joy would be the joy of y’all.”  (2 Corinthians 2:3)

“And besides our own comfort we rejoiced still more at the joy of Titus, because his mind has been set at rest by y’all.”  (2 Corinthians 7:13)

“The grace of the Lord Jesus Christ and the love of God and the fellowship of the Holy Spirit be with y’all.”  (2 Corinthians 13:14)

“For God is my witness, how I yearn for y’all with the affection of Christ Jesus”  (Philippians 1:8)

“We give thanks to God always for y’all, constantly mentioning you in our prayers.”  (1 Thessalonians 1:2)

“The Lord be with y’all.  I, Paul, write this greeting with my own hand.  This is the mark in every letter of mine; it is the way I write.  The grace of our Lord Jesus Christ be with y’all.”  (2 Thessalonians 3:16-18)

Tomorrow:  Yo, She’s Pregnant