Well, that is a quite easy distinction between two totally unrelated plants but what about telling sweet violets (Viola odorata) from the other species of violets often seen? That is a very different issue because they all look "violet-like"! In reality it is not that complicated as there are only four species of violet commonly encountered in the wild and so there is not that many to choose from.
The first and most obvious clue to identification comes in its name, sweet violet or Viola odorata, they have a scent. Secondly they tend to flower very early in the year, even as early as January. Then they are usually on roadside banks and verges, often as a result of garden rubbish being dumped or even where they have been planted; they were a popular garden flower because of their scent and early flowering. Where they grow they can form large clusters or carpets as they spread vegetatively using above-ground shoots. The other obvious difference to the other species is the shape of the leaves; almost heart shaped.
There are other more subtle difference; they tend to be darker in colour that other common violets and they can also occur in white. They also have hooked styles and have hairs on the stem that point downwards but we are now getting down to levels of detail that are the domain of the serious botanist, not the casual observer like me!
So there you have it, that was not too difficult was it? Or may be it was! The thing is that when you look in a field guide you will probably see a number of violets, mine illustrates twelve, but when you take out the rare ones or the ones that grow only in specialised habitats it leaves a much narrower choice and gradually, using different features, the choice becomes pretty obvious.