Is it possible to determine someones league while playing against them?

In theory if you had a person's user name and number, you could look them up (on the blizzard website). Unfortunately those numbers are hidden by default so unless you get your opponent to give you his number, I don't see how.

Ultimately I tend to ask my opponents, and I find this works well. I can probably trust their answers about as well as I can trust them to give their hidden number.

More often than not, I can tell a person's league just by scouting:

• If they have a lot of workers and are using a known build: probably plat/diamond
• If they are doing something weird, or I find a proxy blank, probably gold.
• If they don't have more than 13 workers, definitely silver or lower.

These rules aren't hard and fast, but they do tend to explain 90% of my games.

If I'm getting 4 Gate timing push at 6:30 you can bet the guy is in diamond.

Where this breaks down is when you start to get top 10 diamond and then start seeing things like: bunker push, 2 pylon wall in. And all of a sudden I start sweating...

Here is a picture of the only way I know of determining what rank your opponent is before the game starts. Keep an eye out for the color of the boarder around your opponents unit portrait during the vs loading screen before the game starts.

http://wiki.teamliquid.net/starcraft2/index.php?title=File:Portrait_Borders.jpg&filetimestamp=20140322032611&