There is no defined way of telling the difference. However, there are a few tell tale signs that you could look for.
Firstly masonry paint can look like a plastic coating and have a sheen. It is usually quite smooth and feels plastic when you rub against it.
It can also be prone to cracking, these are usually very fine and will show up if you spray water over the wall. It is also prone to bubbling or peeling off the wall.
A lime wash is usually quite dusty and can be brushed off the wall if rubbed forcefully enough.
Lime wash will also change colour when it is wet, it usually goes darker and you can see it dry out.
One way of telling the difference is to remove some of the paint from the surface. A masonry paint will likely come off in sheets due to its plastic content.
If you would like to discuss this in more detail, please call us on 01208 79779.