Almost all civs are actively "plotting against you" at all times - you can safely assume this to be the case. Only super-peaceful civs like India won't be constantly plotting, but even Gandhi can surprise. If you let down your guard at any point and run with a less-than-average military strength, even your 'friends' will start plotting. Even so, the AI weighs the pros and cons of going to war with you, so even despite this non-stop plotting you won't often be assaulted by your 'friends'. The Civ V world is truly cutthroat, and nobody is ever really your friend. Then again, this is a lot like real-life politics.
As for the OP's question about how reliable this "plotting" information is: It's completely reliable - it's just that for above mentioned reason an attack won't always be executed. Rest assured, however, that if you completely neglect your military strength, your diplomatic relations will fade and war be declared sooner or later. If you start getting many of these "plotting" messages, it means the AI considers you a possible target due to perceived military weakness (perceived as opposed to actual, since you can beat the AI even with a vastly inferior army due to the AI's tactical ineptitude) and you should consider expanding your military if you wish to maintain relations.