Which system version is installed on your Mac?
If you are running macOS 12 or macOS 13, you can easily compare the iPhoto Library to your Photos Library by importing the iPhoto Library into your current Photos Library. When you use the command "File > Import" in Photos and select an iPhoto Library for import, Photos will show you in the Import dialog in the action "New", which photos are not yet in your Photos Library. However, it will take a very long time, if the library is large and may not work at all, if your system is running out of storage. And you do not need to go through with the import - just use it to compare the two libraries and identify missing photos.
I would simply move the iPhoto Library to an external drive, to free some storage and then try to import the moved library. Photos, that are in both libraries will use the storage only once, because Photos is saving storage for identical photos by using hard links, as OT pointed out. This is explained here: Six Colors: The (hard) link between Photos and iPhoto or in Apples support document: Photos saves disk space by sharing images with your iPhoto or Aperture libraries