Edit: No Longer available
The menu has a percentage under the career option to check at any point in the game.
This challenge is really easy even if you play the game on the last day. To get 100% on a track is a gold medal, so if you get all golds on the first 4 events that should get you to 50% overall.
If you can't do that, just try and beat all the tracks you can until you get 50%. If you beat all the tracks you can manage, but still somehow don't have the challenge, go back and try getting higher medals on earlier/easier tracks. This should not be a challenge by any means if you have a few hours and have even the littlest amount of skill with the game.