At the end of standard critical texts of the Greek New Testament, the editor have conveniently add several indices that allow a person to find a OT citation in the NT or find where an OT citation in the NT originates.
These indices are very helpful and they can save one a lot of time. For example if I wanted to find where Josephus quotes the OT, where would I go?
Answer: H. B. McLean, Citations and Allusions to the Jewish Scripture in Early Christian and Jewish Writings through 180 CE (Lewiston, NT Mellen Press, 1992).
Or what if I wanted to find the Scriptures cited or use by a Church Father?
Biblia patristica: index des citations et allusions bibliques dans la littérature patristique.
Don’t let the French title keep you away, it is quite useful with just a little orientation; and the best news! It’s available in a very useful online interface: http://www.biblindex.mom.fr/. For serious work, though, be sure to consult the paper edition and if you do find an error in the online database, be sure to report it.
Are there other indices like these that you know of?