For most of the online VHV repertories, pressing alt-p will load a PDF of the source edition used to create the digital score. The PDF scan of the source edition is opened in a separate window and can be used proof-reading of the digital score.
For example, going to this link:
displays the following digital score:
When pressing alt-p on the above page, the following window will be opened with a PDF file of the source edition:
The VHV editor and source print windows can be resized and arranged for proof-reading: