Intuition, in the common sense of which we are talking about, is basically when a situation triggers the unconscious mind to collect information in the present, and it accesses stored information to contrast and relate it to it, and then bypassing the conscious mind, giving only its conclusion. It seems like magic because the processing is unconscious.
You thought someone was shady because your subconscious mind used information from the present - perhaps it was their body language or facial expression; any number of things. But remember, this is all thought of unconsciously - and contrasted with stored info, and previous thoughts related to it. So someone with similar body language or whatever, from the past, turned out to be shady, and so the subconscious made the connection and voila! A judgement or feeling is thrust into your consciousness.
Intuition, because it is only stored/present information, is only good for things that are already known or right in front of our face (although we might not realize it). One example is that our intuition tells us the Earth is flat, yet it is in reality spherical. For things such as that, things less mundane, it is terrible. For normal, everyday matters, it's great and requires less energy for us than consciously thinking things through. Although perhaps conscious thought allows for a result you can be more confident in.
I do trust my intuition, but I also reason why that might be the case afterwards. Usually it makes a lot of rational sense after I've done so.
