Generally, lovers of Rumi's poetry will write and send a single line from a poem they have read somewhere, usually a poem they were very attracted to, and ask which book it came from. This presents difficulties (unless it is a quatrain-- see below). If the line is from one of Rumi's "longer poems" (longer than four half-lines), then it is from a ghazal, or ode. Unfortunately, there is no data base of such poems available via this website (and even sending the first line will probably not help, unless it happens to be very familiar).
Oftentimes the source of the poem is from one of the various books containing versions by Coleman Barks. Chances are good that the poem has been re-published in Barks' popular anthology of his renditions, "The Essential Rumi," 1995. In this case, it may be found in the index of first lines at the end of the book (even by going to a bookstore, in the case of someone who doesn't have a copy or doesn't wish to buy one).
If the line is from a "short poem" of four (half) lines, then it is probably a quatrain [rubâ`i] (or it may be part of a ghazal). In this case, feel free to send an e-mail to this website and type "quatrain search" in the subject area. Success is most likely if the first line (or even the first few words of the first line) of the English rendering is sent. If not, a computer search of key words may possibly be successful in determining which particular quatrain attributed to Rumi it is as well as the name of the translator or version-maker (with book source and page number). If you want to know if a particular Rumi quatrain you love has been translated or versioned by someone else, this information may also be available.
If you think that what you are looking for may be quatrain, you may be able to find the information by yourself by using the on-line Quatrains Concordances (see article, "About the Quatrains Concordances") If you have a number for the quatrain you are looking for, use either of the two concordances. If all you have is the first words of the first line of the quatrain you are looking for, use the "old concordance" and type in a search for the first two or three words of the first line.
If the poem is a rubâ`i, all that is needed is a transliteration of any of four half-lines. The true "Foruzanfar number" of the quatrain can be given if it is a rubâ`i from his authentic edition (Volume 8). If the quatrain is from the inferior one-volume commercial edition ("Kulliyât-i Dîvân-i Shamsî Tabrîzî," published by Amîr Kabîr), then it may be from the "pseudo-Foruzanfar" edition of the quatrains contained in that volume. Quite a few of these quatrains are not in the authentic Foruzanfar edition. And the ordering and numbers of the rubâ`iyât are completely different in both books.