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How to check that any image was coverted from a color to black and white

Hi wrocek1
No guarantee there will be a trace as metadata can be removed from files. Depending on the post processing software, sometimes some editing history is included in the metadata.
Why not post the original image via a sharing service with a link if you want forum members to give it a look.
Just a suggestion
John Wheeler
Hi wrocek1
The raw data info from Photoshop is given below and indicates the file was created today and there is no trace of being a color file other than the JPEG is in RGB color mode.
Since the file was just created, I am not sure this is the original digital data (unless you just scanned it today) so more information could be obtained if the original file as you received it was shared by link without any resaving. If this the last image is the original digital data, I could not find any traces and maybe another forum member could help.
John Wheeler

<x:xmpmeta xmlns:x="adobe:ns:meta/" x:xmptk="Adobe XMP Core 5.6-c140 79.160451, 2017/05/06-01:08:21 ">
<rdf:RDF xmlns:rdf="http://www.w3.org/1999/02/22-rdf-syntax-ns#">
<rdf:Description rdf:about=""
<rdf:li rdf:parseType="Resource">
<stEvt:softwareAgent>Adobe Photoshop CC 2022 (Windows)</stEvt:softwareAgent>
<xmp:CreatorTool>Adobe Photoshop CC 2015 (Windows)</xmp:CreatorTool>
This is an original https://fotopolska.eu/1352494,foto.html downloaded to my laptop...
Just wonder... if such .jpg was ever converted from full color to black and white..
As I see that image was taken 8 years ago... kind of.. mission impossible to be black and white those days...
If it was taken on negative.. OK.. understand. But it was taken by Pentax K10X
Hi wrocek1
Since the image was taken with a Pentax K10X which is a digital color camera, the information from the sensor in the camera at some point was converted to B&W. I am also assuming that the artist was taking original images from the actual structure and was not taken a picture (e.g. copy stand) of a B&W digital print (which is also possible). I will assume that is not the case.

Where the conversion takes place could be in several places
1) In camera as there is a digital B&W filter in the Pentax K10x. That is still a conversion from the color sensor yet the data coming out of the camera could be B&W
2) Could be done in the Raw processor
3) and of course in a photo editor in post processing

Note that the image that is downloaded has already been post processed as the pixel dimensions do not match the possible format and size options supported by the Pentax K10x. Also, I have the same results in the metadata as you received which indicated the file (included metadata) was created at the time of download.

So the answer being that the RGB information from the camera was converted to B&W somewhere in the workflow.

Since I don't understand the purpose or goal you are trying to achieve with the determination, I am not sure if this truly answer you question(s).
Hope this helps
John Wheeler