This would require JabRef to read metadata from the pdf file, but not all pdf files do have metadata attached. JabRef can read XMP metadata or if you have enabled Grobid, JabRef will try to extract bibliographic metadata from the pdf by analysing and scanning the first page(s) for relevant info. Both are not always accurate, I would suggest to not blindly trust these two methods, especially Grobid. XMP metadata is fine, if you know for sure, that the correct metadata is attached to the pdfs. Instead, I would suggest importing metadata by ID (e.g. ISBN or DOI) yields far more accurate and complete results.
There are multiple methods that allow you to see entries without a file. I will show you two:
- checking the entry table:
- create a group. Go to “free search expression”. Enable “Regular Expression”. Enter
file != .+into the “search expression” field.
- …
If you have an existing library file with lots of entries, it is usually more efficient to use the feature Quality > Automatically set file links (F7). Here are the docs: Manage associated files - JabRef
It’s best to keep the filenames of pdfs and the citationkey pattern similar, so that you can easily detect the pdfs. To avoid duplicate file names, using the DOI or ISBN as filename is great.
You will find I have had a little trouble to find a great regular expression to capture my pdf files too: Help with search expression for 'Automatically set file links' (F7); Attach files to entry, but it’s fine now 
In general, if you have a big library and lots of unidentified pdfs laying around with messy filenames you will have to do a lot of work.
Either you import by “search for unlinked files”, which will import the file and create a separate entry with potentially incomplete metadata in your library, which you then have to merge with existing entries via the duplicate detection feature, or you go the “Automatically set file links (F7)” feature, which would require you to rename all of your pdf files to a pattern that can be found via JabRefs regex engine.