hi, the easiest way to establish that is to ask them the model number, frontlit SPs had the serial number AGS-001, whereas backlit models have the number AGS-101. I belive that the AGS-101 was only released in a light metallic blue and a light metallic blue, along a spongebob version... they were only released in the USA though, and maybe japan.
I ended up buying my two from the USA with crappy shells and re-casing them, the metallic shells seemed to wear really easily so are neigh on impossible to find in good condition, not to mention the colours are
. It was definately worth the effort and expense though, the 101 is the best handheld to play GBA and GBC games on (even better than the DS).
edit: ok, looking at wikipedia it seems a very small number did make it to europe, in dark blue, pink, and "tribal", good luck finding one though.