How do I know when a mallet finger is cured?

It'll stay straight. It takes about 6 weeks for the torn end of the tendon to heal back to the bone. Treatment consists of splinting in extension for 6 wks. The splint must be worn at all times, if the finger becomes flexed at the last knuckle, the 6wks starts over! When the splint is removed, the finger should remain straight.