1. As a verified user
In the Verified App go to the Dashboard or Archive, find the envelope you want and click to expand it. There you can see the statuses for all the signatories, including who opened the documents.
2. As a non-Verified user
If you are a recipient who received an envelope for signing, and there are more recipients besides yourself, you can see who else has opened the documents by clicking the link in the email, opening the document you want and expanding the Transaction log.
NB: if the signing order of the envelope is Sequential, then you will only see those who have opened AND signed the document before you (as you won't get a document until all the recipients before you have signed). However if the order was set as Parallel, then you will see all who opened the document, regardless of who signed it or not.