The Terminal.
You would have to compare the inodes (the entries in the file table) of the original master files.
But the screenshot in your original post strongly suggests that the two libraries are sharing the storage and are not actually using twice the storage - only 72GB for "Bilder". Each library has files of the size ~ 85 GB, but because identical image files in both libraries are using the same disk blocks, the total in file size is much less for the images.
Only the Finder is counting them twice, because the Finder does not know if a files are hard linked. For all practical purposed they look and behave like ordinary files.
If two files are hard linked, the inode numbers will be identical, when you list the file with the ls -li command in the Terminal:
For example, looking into the packages of an new migrated Photos library and the original iPhoto Library. Everything looks duplicated with the same size:
The Terminal is showing this, when I type "ls -li " into the Terminal and drag one of the master files behind this command:
Hermione:~ dreschle$ ls -li /Users/dreschle/Pictures/Photos\ Library\ 2.photoslibrary/Masters/2015/05/28/20150528-184932/IMG_0966.JPG
39167952 -rw-r--r--@ 3 dreschle staff 1283723 25 Mai 12:00 /Users/dreschle/Pictures/Photos Library 2.photoslibrary/Masters/2015/05/28/20150528-184932/IMG_0966.JPG
Hermione:~ dreschle$ ls -li /Users/dreschle/Pictures/iPhoto\ Library.migratedphotolibrary/Masters/2015/05/28/20150528-184932/IMG_0966.JPG
39167952 -rw-r--r--@ 3 dreschle staff 1283723 25 Mai 12:00 /Users/dreschle/Pictures/iPhoto Library.migratedphotolibrary/Masters/2015/05/28/20150528-184932/IMG_0966.JPG
In both libraries are the files listed with the identical inode number 39167952 (shown in red in my screenshot) , meaning they are linking to the same physical file in the file table. For the Finder both files are separate, regular files and so it is reporting the size twice.
When I compared the used space on my hard drive right after migrating the test library with 40 photos, the used storage had not been increased much:
Re: Can I delete my old iPhoto library after migrating data to iphoto in iCloud?
Delete the iPhoto library anyway, if you do not need it any longer. It will free some storage. But you need to free storage urgently, if you only have a bit more than 5GB free. That is not safe.