How do I find out the jpeg quality of a jpeg?

[PAEz]

I kinda hoped that ImagingJpegQuality would update if I opened a jpeg file but it doesnt and Ive looked around but cant figure it.
So how can I find out what quality an image is using after I load it?
I know how to set it (nice and easy) before a save. but Id like it to match the original if the user doesnt pick a different setting.

[Galfar]

Jpeg quality is just for saving and it's used for quantization of DCT coefficients - basically it tells the Jpeg compressor how much information to throw away (lossy compression).
There's no original if the image is saved as Jpeg - each time you save it the image will lose
some information.