Is there a way to see how my rating will be affected BEFORE I play someone?

You can do this by using the 'Assess' command.
Assess shows you the rating changes that will occur if you win, lose or draw against another player.

BlitzIn shortcut:
Highlight a player's username, right click, then select 'Assess' from the right click menu.
OR: Right click on a dot in the seekgraph, then select 'Assess' from the right click menu.

Dasher shortcut:
Right click on a player's username in a channel or chat, then select 'Assess' from the right click menu.
OR: Right click on a dot in the seekgraph, then select 'Assess' from the right click menu.
OR: OR: Right click on a username in the list of people in a channel, then select 'Assess' from the right click menu.

Command Line Method:
Type 'assess username' in the command console.
To see the rating changes between two other players, type 'assess username1 username2'
To see the rating changes in a game you are currently playing, just type 'assess'
'Assess' also lists the current rating of each player, in each rating category.