Rashi to 45:4 says that when Yosef told his brothers to approach him, it was to show them his circumcision (presumably to prove he was Jewish). But how would that prove anything? All of Egypt had been circumcised already, as Rashi says on 41:55. If his showing his circumcision wasn't to prove he was Jewish, why did he do it?
Chizkuni asks this and offers two answers:
The reason the Egyptians were circumcised was because of the hunger of the famine. Yosef however was rich and therefore the only reason he would circumcise himself would be if he was Jewish.
Although all the Egyptians were in fact circumcised, the brothers were not aware of this this and would recognize Yosef on the basis of his bris.
The Tzeida Laderech adds two more explanations:
Since Yosef was born circumcised (as Midrash Rabba says), this would be apparent.
Our sages say that one who has relations with a gentiles affects his bris, so seeing his bris proved he was not Egyptian.
In the הסכמות to the sefer נחמד מזהב, Rav Shmuel Yaakov Koppel Katz says that from the time that Avraham was commanded to do Bris Milah until Har Sinai, all of Bnei Noach were commanded in this mitzvah, but they were not commanded to do priah (the uncovering). But Tosfos writes that Avraham himself did do priah since Yisrael were to be commanded to do so in the future, and his offspring certainly continued to do this as well.
This is what Rashi means when he says "שהרי אני מהול ככם" - I am circumcised like you, with priah, not like the Egyptians who have not done priah.
If the brothers knew about it, they would have known the reason - that they were forced to do it in order to obtain food.
Yosef, as the second in command, would not have been forced due to his position (giving out the food to everyone else). So the fact that he was circumcised would have had to have been for a different reason.
The Taz asks this question in his Divrei Dovid and answers that Yosef had not actually attempted to have the Egyptians circumcise themselves, he was merely proving to them to what extent they were required to heed his every command. Then afterwards there was no circumcision carried out.
Kli Yakar explains that one who has relations with an Aramis (gentile) his Orla gets stretched (Moshcha Orloso). He wanted to show the brothers that he is the same Tzadik and did not sin while he was in Egypt.
Just a guess, but probably the wound having healed from childhood would look very different than an adult's fresh one.
Ok. Heres another answer. This one is good. The egyptians hated circumcisions, it was a cherpa to them, (as found in Etz Yosef in the name of Yaaros Dvash). For a person to happily show his milla would be a proof that he is a 'Mahul' and not just a person missing his foreskin.
Yosef made this rule two years into the hunger. That is the same time the brothers came. Perhaps the rule was after they came.
And an even more intriguing approach is that he didn't literally show them a body part, but rather proved to them, and showed them that he is Mahul, precisely with this ruling.