Look at the vertical pin the lever pivots on - if it's hollow, there's a spring clip on the bottom that holds the pivot in the master cylinder body. the clip can fall off pretty easy, and the pin can then work its way out, and the lever will just fall off.
Being that the pin is hollow, you just run some wire through it to secure the pin to the master cylinder body.
If the lever's pivot isn't hollow, and is secured by a bolt - you're fine.
If you're in doubt, ask someone in the pits, or a staff member - it's a two minute fix that someone will be able to help you with...