Another thing to note, for 2007-2009 the Flex-Fuel badge was yellow. In 2010, the Flex-Fuel badge became green. This is a commonality among all GM vehicles with Flex-Fuel badging.

The grille is not the easiest detail to find but it's a noteworthy one.

nice, i knew for the chrome strip and the C-pillar badge but not the grille

Since we have a massive amount of 2006-12 Impala's, I found some information that would make identifying them more useful.

For 2006-09, there was an Impala badge on the c-pillar that was later removed for the 2010 model year.
2006 - http://gtcarlot.com/colors/car/12048520.html
2010 - http://gtcarlot.com/colors/car/28802218-2.html

For 2007, the FlexFuel badge was added to the rear trunk (as pointed out by rjluna)
http://gtcarlot.com/colors/car/29097432-4.html

This applied to all trim levels.

2012-13 had it's own distinct grille.
http://gtcarlot.com/colors/car/53621602.html (chrome)
2006-11 grille - http://gtcarlot.com/colors/car/4012273-4.html

Also, 2012-13 didn't have a chrome strip on the trunk compared to 2006-11.
http://gtcarlot.com/colors/car/56789365-7.html
(2006-11) - http://gtcarlot.com/colors/car/4012273-3.html