Generally for a sport bike, 20,000 miles is comparable to 150,000 miles on a car. If the bike has aftermarket parts, expect that it has been ridden hard here and there.
If it's 100% stock, your chances of a properly maintained bike are increased in my opinion. But again, even a fully stock bike could have been ridden to death.
Confirm that there is a stock body kit on it, not aftermarket cheap Chinese parts. Look for scratches on the sides to see if it's been dropped/layed down.