You can actually query an estimate for the quality setting used to save a JPEG image.

In FileMaker: 

Set Variable [$img; Value:MBS("GMImage.NewFromContainer"; Image::InputImage)]

Set Variable [$value; Value:MBS("GMImage.GetAttribute"; $img; "JPEG-Quality")]

Show Custom Dialog ["JPEG Quality used to compress …"; $value]

Set Variable [$r; Value:MBS("GMImage.Destroy"; $img)]

Or in Xojo:

dim f as FolderItem = SpecialFolder.Desktop.Child("test.jpg")

dim g as GMImageMBS = new GMImageMBS(f)

dim s as string = g.attributeValue("JPEG-Quality")

MsgBox "JPEG Quality: "+s

GraphicsMagick lets you read various attributes including EXIF, IPTC and 8BIM metadata.

The JPEG loader in GraphicsMagick checks the JPEG settings for colorspace, orientation and quality, so the same settings can be used to save the same image again.