That depends ...
There are 2 common file extensions for Excel spreadheet files: the older .XLS files and the newer .XLSX files (Excel 2007 and newer)
There are existing Windows methods - that Everything uses too - to intelligently read the contents of spreadheets, documents, PDF files etc.
But the names of the sheets are not part of what Windows will read.
(Not entirely sure; I don't use Excel or Windows Search, but this is how it should work)
The newer XLSX fileformat is basically a zip-file. You can extract it's contents with any compression utility. The result is a couple of XML files in some directories.. (If you do this, you will notice that the sheet names are even in a separate folder from the sheet contents)
Everything has an EXTRA option to read the content of files if the Windows methods as descibed above don't work or are not available: it can also read (what I call) "raw data". Start from bit 1 and stop at the last bit to find the text (or sooner if the text was already found)
But that doesn't work on XLSX zip-files: it's content gets "encrypted" during the zipping.
So, no luck with XLSX files ...
XLS files on the other hand are a lot more straightforward. You can probably find the files containg the 'Backward' sheetnames with a command like:
shell:desktop ext:xls content:Backwards
The "intelligent" still will not find the sheetnames, but the "raw" method will.
Be aware that this can take a while, as Everything has to read each XLS file completely on-the-fly (twice; first using the "intelligent"method and then the "raw" method).