With regard to the fixed camera sites, they are clearly marked on route planners but are also "advertised" as you approach them, you can't miss them either on the road side, very large and very grey. So if you get "flashed" by one of these you deserve it.
The real game is spotting the mobile ones. Last year, returning to Cherbourg on a very deserted country back road, me and a black Chevy in close formation were pushing on a bit up a hill only to come onto a well disguised mobile trap at the top of the hill, loads of gendarmes milling about, fortunately the trap was directed the opposite direction to our travel, what a relief.
of course the gendarmes setting the traps have all attended the advance urban camouflage course run by the foreign legion, they're good in fact very good.
best of luck