Quantitative traders know what their edge is through testing historical data. They know which patterns or programs are showing promise. Of course they don't know if they have a real edge until they start making consistent money.
A discretionary trader usually does not know what their edge is because it is a combination of the mind, the gut and the instinct reacting to patterns and other information. They only know they have an edge if they are making consistent money. The TA they use cannot be measured because out of 10 similar patterns, they may only take two. And their bet size may vary or their stop may vary depending on the trade - traditional money management is dynamic.
For quants their edge could disappear overnight if the market changes. All that work and money down the drain. The rigidity of their systems is their weakness as it is hard to tell the difference between a draw down and the end of your edge.
Discretionary traders adapt to changing markets better, but their weakness is maintaining a peak mental state that is not disrupted by lack of focus, lack of confidence, over confidence, emotions etc., which can discombobulate one's mental machine.
The bottom line is, if the quant method is your game you can find an edge through testing, but you will only know if your edge is real when you are making money. For a discretionary trader, you will only know if you have an edge if you are making money. If you are discretionary, don't waste time worrying about what your edge is - just make money or die trying.