I cut my thumb with a knife opening a package how do i know if i need to see the doctor?

Profuse bleeding: Numbness beyond the cut tetanus over 5 years inability to flex or extend.

See a doctor: If it a paper cut like injury - probably don't have to see a doctor. However, if you have to ask, you probably should see a doctor. There are many nerves, tendons and ligaments that can get injured. A proper hand examination by an experience physician, can usually rule out any any injury.