Generally speaking hail of one inch or greater can damage a roof to the point of warranting a replacement.  While hail damage usually does not cause an immediate leak or an obvious amount of visual damage , it is important to know that your roof has been comprised in a hail storm and it is worth having checked out by an industry professional.  When a hail stone impacts a roof its kinetic energy travels through the shingle to its bottom side and underlayment, creating tears or micro tears in the laminate.  This only worsens with time and change of temperatures, thus compromising the structural integrity of the roof.  On the surface however, the impact will usually only appear as only a small spot on the shingle where a few granules are missing.  A heavily damaged roof will have many granules in the gutters and downspouts as well.   In extremely severe cases decking may be damaged.  Unfortunately, many homeowners don’t realize their roof has hail damage.  A proper inspection must be done and can not be done without getting on the roof.  Unfortunately naivety and hubris can lead some home owners to insist their roof has no damage.  Some may state”my roof is fine” while standing in the yard with no roofing or insurance adjusting experience, only to learn months later after a leak that their window to file a claim has passed, and that what could have been covered by their insurance must now be paid for out of pocket.  Most roofing companies offer free inspections so there is no reason to not get a third party opinion from a trained professional who can tell you if your roof warrants replacement or filing a claim.