As a bochur, Rabbi Sholom Blank, approached the Rebbe to receive a bottle of mashke and extended his left hand. A nearby chossid told him to use his right hand instead, but the Rebbe knew better.
Rabbi Sholom Blank, shliach to Miami, Florida, relates:
During my bar mitzva yechidus in 5717 (1967), the Rebbe asked me if I knew how many strings were on my tzitzis. I answered that there are thirty-two strings total. The Rebbe then took out a piece of paper and wrote down the number 32, using the letters lamed and beis, and then explained that this spells lev, meaning “heart,” and gave me a bracha to have a gutte hartz, a vareme hartz, un a Yiddisher hartz (a good heart, a warm heart, and a Jewish heart).
During one farbrengen when I was a bochur at 770, the Rebbe was handing out bottles of mashke, and called out, “Who is going to Montreal?” Since no one answered, I said that I was.
The Rebbe reached out to hand me the bottle, and I stretched my left hand to take it. As I did that, the elder chossid Reb Yisroel Jacobson called out, “Take with your right hand!”
But the Rebbe turned to him and said, “No, he’s left-handed.” The Rebbe knew that I was a lefty and that, for a lefty, the left hand takes the place of the right hand. The Rebbe knew me.
From The Weekly Farbrengen by Merkaz Anash