You get the Win percentage by running your hand against a perceived range, using a (free) tool like Equilab.
e.g. Suppose you had AhAs on 8h 7h 4c and you think villain will play for stacks with straights (65s), sets (88, 77, 44), overpairs (JJ-99) and a handful of flush draws (KQs-T9s), then Equilab can calculate your equity for you.
Board: 8
7
4
*******Equity*****Win*****Tie
Hero **56.56%**56.02%***0.54%*{ AhAs }
Vill **43.44%**42.90%***0.54%*{ JJ-77, 44, 65s, KhQh, QhJh, JhTh, Th9h }
56.56% would be the number you use for the %W in the EV calculation. (And 100 minus 56.56% would give you the %L. You could also just look at villain's equity, which is 43.44% in this example). Since you have more than 50% equity against that range in that example, you don't need to know the precise EV. Being a favourite automatically means it's profitable.