Exactly. While the majority of colubrids do posses venom glands (formerly viewed as unique organels - Duvernoys glands - but now taken at face value again) there are still some that don't. Additionally, many venomous colubrids do not posses the modified dentition be be classed as rear fanged. For example, in the US Hognosed snake and Lyre snakes are rear fanged and venomous. The coachwhips are venomous but not rear fanged, while the American ratsnakes, kings, milks, gopher/bull snakes are non-venomous. The ratsnake issue can cause confusion as many Asian species are, indeed, venomous. But these are taxonomically very distinct from the American snakes of the same name.
A good bit of background info into this can be found on this PDF:
http://biology.bangor.ac.uk/~bss166/Publications/Colubritoxin.pdf
In the US, the only dangerously venomous snakes are the vipers/rattlers and the coral snake. All of which are easy enough to ID most of the time (biggest risk for confusion being corals/milksnakes and cottonmouth/water snakes).
The diamond shaped head division between ven. and non-ven. is woefully vague and is best forgotton as it doesn't work.
The snake pictured is indeed a ratsnake,
Pantherophis obsoleta (Formerly
Elaphe obsoleta).
Regards, Lee.