How can I see the captured pieces and material count?

Underneath each player's username you can see which pieces they've captured. If there is a material imbalance, this will be reflected with a number next to the pieces of the player who has more material left on the board.

Only the player who is ahead will have the + number, the other side will show no number. 

Note that in the above image, Dave is playing with the black pieces. The white pieces under the player name are the pieces that they have captured.