Determine the quality level of JPG images and other photo properties
This online service will show the quality level of JPG images.
For lossless compressed images, the image compression level will be shown.
In addition to the compression level, information about the number of unique colors in the image, color depth, image size will be shown. Detailed information about each color will also be displayed.
JPEG is a lossy format. Every time you save the same JPEG image, regardless of the quality level, the actual image quality is reduced. So even if you get the quality level from the file, you won't be able to keep the same quality when resaving the JPEG (even with quality=100).
Low quality JPEG files do not simply become more blocky. Instead color depth is reduced and the detail of sections of the image are removed. You can't rely on lower quality images being blocky and looking ok at smaller sizes.
See the article “How to check JPG quality level” for details.