When using a DSLR, checking RGB values (which range from 0 to 255) are a great way to check exposure. Individual RGB values that approach 255 mean those pixels are approaching white - there is limited detail available in those areas. For very small areas (like a highlight on an eyeball) high RGB values may be acceptable. Generally though, it is a good idea to keep values under 255 so you have options to adjust exposure later in editing software. Conversely, values approaching 0 mean the area is dark, with limited detail available.