A function included in the GECK version 220.127.116.11.
Gets the highest completed quest stage. For example, if stages 10, 30, and 75 were completed, GetStage would return 75. Even when stage 30 is completed after stage 75, GetStage will still return 75 (i.e. the highest one). If you need to know if a certain quest stage is completed, use GetStageDone instead.
(int) GetStage questID:form
Link to TES4 Construction Set Wiki: GetStage.