As @Kushaj said, True color RGB images will use a bit depth of 24 (8 for each channel).
However, your images can of course come from another domain, which might use another bit depth value.
E.g. depth images often use a single channel with 16 bits, while Dicom images might use 12 bits.
If you can load the image via PIL, you could check the image.depth or image.mode attributes.