I heartily concur with Philip Brown! What an excellent idea! I have toyed with doing such a comparison on my own using the word list generator. There, I would generate a list of words from one text and compare it to the word list of another text. I would then do a limited search of ONLY the two texts using the list of words COMMON to both texts as an inclusion list. Then all vocabulary that is similar in both texts is highlighted.
Here's an example of the results with Isa 40:6-8 & 1Pet 1:24-25 that proves the method potentially helpful (I marked the words that were not directly involved with the quote in blue. The red words are the common vocab, the black words reveal the differences in Peter's quote of Isaiah):
Isaiah 40:6-8 fwnh. le,gontoj bo,hson kai. ei=pa ti, boh,sw
pa/sa sa.rx co,rtoj kai. pa/sa do,xa avnqrw,pou w`j a;nqoj co,rtou
evxhra,nqh o` co,rtoj kai. to. a;nqoj evxe,pesen
to. de. r`h/ma tou/ qeou/ h`mw/n me,nei eivj to.n aivw/na
1 Peter 1:24-25 dio,ti
pa/sa sa.rx w`j co,rtoj kai. pa/sa do,xa auvth/j w`j a;nqoj co,rtou\
evxhra,nqh o` co,rtoj kai. to. a;nqoj evxe,pesen\
to. de. r`h/ma kuri,ou me,nei eivj to.n aivw/naÅ
tou/to de, evstin to. r`h/ma to. euvaggelisqe.n eivj u`ma/jÅ
The problem of course, is that this does not take into account word order! So, when all of this said and done, it would be just as easy to visibly compare the two texts!
So, Philip Brown's idea remains the answer!