The Mishna in Sotah 1:5 states
וְכֹהֵן אוֹחֵז בִּבְגָדֶיהָ, אִם נִקְרְעוּ נִקְרָעוּ, אִם נִפְרְמוּ נִפְרָמוּ, עַד שֶׁהוּא מְגַלֶּה אֶת לִבָּהּ, וְסוֹתֵר אֶת שְׂעָרָהּ. רַבִּי יְהוּדָה אוֹמֵר, אִם הָיָה לִבָּהּ נָאֶה, לֹא הָיָה מְגַלֵּהוּ. וְאִם הָיָה שְׂעָרָהּ נָאֶה, לֹא הָיָה סוֹתְרוֹ: And a priest grasps her garment--if it tears, it tears; if it unravels, it unravels--till he has bared her bosom, and he loosens her hair. Rabbi Yehudah says: if her bosom is beautiful, he does not bare it; if her hair is beautiful, he does not loosen it.
My question seems rather basic but I did not see any of the standard commentaries (Bartenura, Rambam, Tosfot Yom Tov) address it.
The question is according to R' Yehuda if her bosom is beautiful the Kohen doesn't reveal it. How does the Kohen determine this, especially if it needs to be determined before he rips her clothes and bares her bosom?