Probably the cheapest chrony check is some .22lr ammo. Ideally you buy a bunch of one lot, check it on several chronographs and then use it for yuor own chrony check for years to come. A more slapdash method is to take several different types of .22lr and shoo it over your chrony . If the velocities are all way high or way low from published specs for your barrel length then you know you have a problem-If the .22 stuff is reasonably close then your chrony can be presumed to be reasonably accurate also.