Using ImageMagick ($ brew install imagemagick) certainly helps. E.g.
$ identify -verbose baseline.jpg | grep Interlace Interlace: None $ identify -verbose progressive.jpg | grep Interlace Interlace: JPEG
As you can see baseline.jpg is baseline, i.e. not progressive, while progressive.jpg is indeed progressive.
Newness
Apparently in newer IM (v7.0.3.8+) you should be able to tell without resorting to grep, like so
$ identify -format "%[interlace]" progressive.jpg
See https://www.imagemagick.org/script/escape.php
A quick reminder
You can use jpegtran (or MozJPEG) to convert to and fro the baseline/progressive.
In MozJPEG the -progressive option is default, so you need -baseline apparently (did not work for me with the brew install of MozJPEG). In jpegtran baseline is default, so you need to say -progressive, as in
$ jpegtran -progressive in.jpg > out.jpg
