I don't think it's possible to calculate EV exactly while playing online. I mean, you could have PokerStove open and all, but I'm pretty sure it would be more of a distraction than anything else.
Here's a example that I reviewed just some hours ago, check the hand and then get here and fire up PokerStove.
So I had Qc8s and flop came 2d 8d Qs. I didn't know what he had and I had a top 2 pair. I bet he called. I put him on any suited connector, any queen, any broadway with a 8 and any pair.
If you evaluate my hand in this flop against that range that gives roughly 85% win for my hand.
The turn completed the flush, so yeah in a way that worried me but didn't change the range since I bet and he called, although now there was also a straight draw. Then the river confirming the straight, but again, his range didn't change much. Him calling my bets never really changed the range I put him on.
But putting the whole board on PokerStove gives 82% to my hand. He had $0.43 left and the pot was $1.86 so EV was:
0.82 x $2.29 - 0.18 x $0.43 = $1.88 - $0.07 = $1.81
Almost a full buy-in worth of EV it's a real easy call.
Let me be honest, I really thought his range through, but I did not calculate EV during the hand, just that I was ahead against the range.
Funny though, I didn't put the actual hand he played in the range (T9o) although I had T9s, but it was close enough. I don't even understand what the hell he was chasing to be honest.