A useful website given the often fragmentary nature of Coptic literary texts: https://atlas.paths-erc.eu/.
At the moment, I’m using it mainly to check if additional fragments of a work I’m reading have been preserved. For instance, yesterday I was browsing the ÖNB collection and read the apparently hitherto unpublished remnants of the Martyrdom of Pekiosh.
PAThs makes it possible to check for additional fragments in other libraries. First by using the Works section of the website and searching for the work’s title.
Once you’ve opened the work’s page, there is a list of every relevant manuscript on the right side.
Clicking on one the manuscript reference opens the page dedicated to this manuscript, in which there is a list of all known fragments (under “Shelfmarks”). In this case, additional leaves of the manuscript are preserved in the BNF Copte 129 (16) volume, which is easily available online.
Now, PAThs seems to be very much manuscript centered. That is, it makes it easy to reconstitute codices, not so much individual works. That’s because the page dedicated to an individual work does not (as far as I can see) contain a list of the relevant fragments, only a list of the relevant manuscripts. And since one manuscript can contain several different works, the list of fragments given in the Shelfmarks section of the manuscript page may or may not be relevant to the specific work you are looking for.