How would I know if I need a tetanus shot if I have a laceration?

Depends. For those who completed the primary tetanus series & have had a booster within 10 years, cleaning of a simple fresh wound & proper closure would be ok. If a dirty wound, soil or rocks etc . > 5y from last booster would leave you at some risk. If the primary series was never complete or you are past the protected zone, a booster and infiltration of the wound with antitoxin might be needed.
If you've never had. If you've never had a tetanus vaccine or it has been ten years since your last one.