1st I beliebe that all JPG are RGB, aren't they?
anyway what about loading the image and check it's color type?

mmh.... maybe you don't know how?
here is an idea of pseudo-code-solution
bool IsMyImageType(Stream data)
{
   try
   {
       using(Bitmap bmp = new Bitmap(data))
       {
           if(!bmp.ImageFormat.Equals(ImageFormat.Jpeg))
               return false;
           switch(bmp.PixelFormat)
           {
               default:
                   return false;
               // as far as I believe only the 1st case is really needed
for a JPEG file
               case PixelFormat.Format32bppRgb:
               case PixelFormat.Format16bppArgb555:
               case PixelFormat.Format16bppRgb555:
               case PixelFormat.Format16bppRgb565:
               case PixelFormat.Format24bppRgb:
               case PixelFormat.Format24bppArgb:
               case PixelFormat.Format32bppAgb:
               case PixelFormat.Format32bppPArgb:
               case PixelFormat.Format48bppRgb:
               case PixelFormat.Format64bppArgb:
               case PixelFormat.Format64bppRgb:
                   return true;
           }
       }
       return bm
   }
   catch { return false; }
}