Answer:
   
   Inmates who were Jewish before coming to prison can usually give
   references from relatives, and if they were affiliated, from rabbis.
   One could also inquire as to whether they kept kosher before going to
   jail and how observant they were. Additionally, their observance level
   should be apparent by other practices in prison (i.e., do they attempt
   to observe the Shabbat?). In the UK, the official line (set by the
   prisons) is that if the inmate ate Kosher food outside prison, he/she
   has a right to it inside prison. There is at least one progressive
   rabbi in the UK whose rule is: If a Jewish inmate requests Kosher
   food, he will approve it.
   
   There are, of course, prisoners who convert to traditional Judaism.
   Most non-Jews who chose to convert need a Rabbi for guidance and, of
   course, the conversion procedures (circumcision or drawing of blood if
   already circumcised and immersion). Those who are Jewish and want to
   become more observant, can take the steps slowly and under the
   guidance of a rabbi, as well. If they are truly sincerely, they can
   begin with a vegetarian diet, and demonstrate their sincerity thourhg
   other Jewish practices. For example, do they keep the Sabbath, do they
   pray (three times) daily, do they have and put on Tephillin, do they
   engage in serious Jewish study, have they set about to make "Tshuvah"
   repentance for those they harmed that might have caused them to be
   incarcerated. Do they even know what Tephillin is?
   
   However, the issue is complicated. There is a halachic ruling about a
   person who claims to be Jewish. If he makes the claim outside of
   Israel, you accept him at his word since there is no advantage to be
   Jewish in the Diaspora. If he says he is Jewish in the land, you must
   question him if you are suspicious since there are advantages to be a
   Jew in Israel. The same should hold for a Jew in prison. Is there an
   advantage to be an observant Jew in prison? Would he get special
   privileges? If so, question him.
   
   If the party in question is a convert, ask him for the rabbinic court
   that presided at his conversion. What did he have to do for
   conversion? If it is a legitimate rabbinic court, there would be
   records. Ask what synagogue he was a member of before prison. You can
   contact that synagogue and they should know him. Ask him who was his
   rabbi? Ask if his family can verify his Jewishness and level of
   observance. Ask him if his mother was Jewish? Jewish diet is important
   but not the most important part of being a Jew.
   
   There are also a number of rulings that affect Jewish practice in
   Prison. In Ross v. Coughlin 669 F. Supp 1235 (S.D.N.Y. 1987), the key
   points were (1) that a prisoner can wear a beard but should shave for
   an ID photo; (2) yarmulke, tallith & tallith katan are allowed; and
   (3) Kosher food is allowed. Young v. Lane 922 F. 2d 370 (7th Cir.
   1991) also ruled that a prisoner may wear yarmulka Ward v. Walsh 1 F.
   3rd 873 (9th Cir. 1992) was a case where Kosher food was requested,
   but the request was remanded to District Court for more facts. Candles
   were not allowed because of security. Transferring the prisoner on
   Shabbat was permitted, because forcing prison not to transfer would be
   too much of a burden. There was also no obligation to get a rabbi. In
   Best v. Kelly 879 F. Supp 305 (W.D.N.Y. 1995), the prison chaplain
   said the plaintiff was not Jewish, and the court held he could not
   wear a yarmulke. In Thomas v. Lord 174 Misc.2d 461, 664 N.Y.S.2d 973
   (N.Y. Sup. Ct. 1997), No. 1963-96. Dated: July 8, 1997, a prisoner's
   request that court declare her a member of the Jewish faith and that
   prison authorities accept her as such was denied; however, the
   non-Jewish prisoner had a right to participate in all Jewish religious
   observances to the extent allowed by the teachings of the religion and
   subject to any legitimate or penologic restrictions that may be
   appropriate. In People ex rel. Sarkis 175 Misc.2d 433, 668 N.Y.S.2d
   435 (N.Y. Sup. Ct. 1997)a petition by an Orthodox Jew acquitted of
   second degree murder on ground of insanity and committed to
   psychiatric hospital to be furloughed during Jewish holidays was
   denied, as were his proposed alternatives to the accommodations being
   made by the hospital. In Umar v. Scott , 991 S.W.2d 512 (Tex. App.
   1999), the prison policy of not allowing inmates to grow beards,
   except for legitimate medical reasons, along with the policy of not
   allowing closed custody inmates to attend congregational religious
   services or religious classes together was ruled as not violating
   Muslim prisoner's free exercise rights and equal protection rights
   under the U.S. Constitution, the Texas Government Code, or the Texas
   Constitution.