GST Reverse Charge

What are the things to consider to know if a service is covered under Service Tax or GST? For example an RCM applicable service is provided after July17 but an advance was already received in say May 17. 

So should we see for Point of Taxation Rules under Service Tax ? Or what should I do to know if a service is covered under GST or Service Tax.

Thank You 

Please share what you can think of this situation.