There is a game on Lumosity.com called Chalkboard Challenge. It gives you two expressions and you have to quickly judge which is larger. The better you do, the more time you are given to play. There are often tricks you can use so you don't have to exactly compute the value of each expression. I find the game tests my brain more than anything else I do. My results are strongly correlated with my brain "strength" at a given moment.

You have to pay for a membership to play.