I have a standard jpeg image, which I use within some commercial software to colorize other data (by mapping the image's color onto the data). Then I export the colored data from this software to an XYRGB ascii file, i.e. I store the data information in the first two columns of each row and then the three RGB colors in the last three columns.
Since I need to convert the color to CIELab or CIELuv, it seems I need to know which exact colorspace (RGB, sRGB, gamma, whitepoint - you name it) my RGB values are in. But the question is: How can I find out? Or could I just assume a certain profile being a good approximation?
(Remark: The company of the commercial software I used was not able to tell me any specifics...)
If you don't know the provenance of the image, there's not anything you can do to determine the color space from the RGB data alone. It's a little like having a blueprint without a scale. You could guess and check with an application like Photoshop that can assign a profile to an image but even then it's not always obvious which is correct unless the image contains colors you can recognize as correct.
For many images sRGB is good guess. Most image on the web are sRGB and many non-color managed apps assume sRGB. But just understand that it is still a guess. If color accuracy is critical, you need the profile.