I had it that the flow of mana is something you can't see, but if you know what you are looking for you can see its effects. In the same way, if you and a trained and experienced meteorologist both look at the sky, you both see exactly the same thing... but the information you glean from what you see is likely to be vastly different.
Examples: When someone who can sense magic looks at a piece of ground and sees a straight line where a plant that is typically found only in high magic areas is suddenly common and lush, fading out to either side both in numbers and health, he considers it likely there is a ley line there. You see the exact same thing, but not knowing its significance, you draw no conclusions from what you see.
Someone who can sense magic looks at a Personal mana spellcaster and notes that his hair does not move with body motion and local breezes in quite the same way as that of everyone around him. Nothing impossible, but less probable than usual. In other words,
extremely minor Fortean effects. When he begins casting a spell, the tendency for local items (tree leaves, leaves on the ground, dust) to act in ways
not quite natural to the local environment extends itself some distance around him. Say, every tree within 25' loses half a dozen leaves
at precisely the same moment. The effect is quite subtle, so if you aren't looking carefully you won't notice it at all (which accounts for the use of normal Perception skill, but at a penalty). Even if you do notice, if you don't have the knowledge which tells you of its significance, once again you draw no conclusions from what you see, while he does.
See what I mean? It has nothing to do with knowing about atoms, or the scientific method, or anything like that. It's just the ability to infer things from what you see that those without your knowledge do not. The basis of it in the case of my world setting is the assumption that all magic of any kind has what I call "splash." That is, any magic alters the structure of local reality to some degree, causing things that are to some degree improbable
given the local baseline of reality to become commonplace, in direct proportion to the degree of alteration. In other words, Fortean effects. Since any spellcaster takes care to try to alter reality only to the degree and in the manner he wishes, normally such effects are very small and aren't noticed unless you are both paying attention and know what you are looking for. They only exist at all to the degree the caster's control of reality is less than perfect. However, reality being huge, complex and interdependent, nobody's control is 100% perfect, there will always be unintended consequences of some sort. The trick is being able to spot which is the normal chaos of local reality and which is induced by magic use.
Of course, when a spell
fails, all that can no longer be taken for granted. In addition to the "normal" effects of failure listed on the chart, a spellcaster in a temperate forest during high summer who fails a casting may find 50 pounds of snow falling on him out of a tree. The worse the failure, the more improbable the effects will be in context of local reality.
Just because it's "magical" doesn't mean it mustn't make any sense.