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This question explains that the extra hair emanating from the tefillin shel rosh of Rabbeinu Tam tefillin is a way to identify the shel rosh of Rabbeinu Tam tefillin vs. Rash'i tefillin.
If 2 tefillin shel yad were next to each other, one Rash"i and the other Rabbeinu Tam, how could one distinguish them from their exteriors, if at all?
With regular sized tefillin I am not aware of any exterior difference between a Shel Yad according to Rash"i and one according to Rabenu Tam. With small ones (called Pitzponim) where you wear two sets (one Rash"i and one Rabenu Tam) of small tefillin at the same time, there is one difference. Though this is not a strict requirement but only the common custom: The retzu'ot of the Rabenu Tam are a tiny bit more narrow then those of Rash"i. That is because the Rash"i ones need to cover the Rabenu Tam ones. Both for the Shel Yad and Shel Rosh. Also the batim could be a tiny bit smaller. But this only applies to pitzponim where you wear both at the same time.
On the shel yad no you can not tell Rashi and Rabbenu tam, but lots of sofrim make the Rabbenu tam Bayis smaller than the Rashi so you can tell
Some sofrim cut the retzuos of Rabbeinu tam tefillin to a peak instead of the common slant on Rashi (^ shape vs /). I wish this convention was more common, for example I've never seen it on Lubavitch tefillin and Lubavitch has the greatest demand for Rabbeinu Tam tefillin by far. I've also seen people write ר״ת on the underside of the מעברתא of the Rabbeinu Tam set, but both solutions require the last person who saw the shel yad open to indicated that it was Rabbeinu Tam.
