the most commom method is to multiply the win dividend of your 2 runers then devide by 2
example

goulburn race 2 the winner was paying $3.60 the second horse was paying $7.70 unitab
$3.60x$7.70/2=13.86
quinella paid $18.60
so the quinella paid better than expected

you can do the same for an exacta but deviding by 1.2

don't forget this is subject to the price of the fav if it wins and the size of the pool...i also have one for trifectas that is just as accurate if you want it