You can view NAGS eligibility report at this link.
Swimmers, who wish to enter 50m events, must have achieved a qualifying time in the 100m or 200m of the particular event and also achieved the published qualifying time for the 50m event. E.g. 50m Butterfly, the swimmer must have qualified in either the 100m or 200m Butterfly and achieved the 50m Butterfly qualifying time. A swimmer that qualifies in the 200m IM event may enter one 50m event of their choice as long as they have achieved that 50m events qualifying time.