If there is no sale agreement executed between buyer and the builder then he cannot demand service tax, because this will go unaccounted and you may have to pay it once again afresh on a fresh demand at a later stage.
Buying a flat can fall into two different categories: a) when the building is under construction, and b) when the building is completed.
When a building is under construction, the Maharashtra Ownership of Flats Act (MOFA) applies. This requires the builder to enter into an agreement in a specified form and attach a Commencement Certificate (CC), which in Mumbai means the IOD (Intimation of Disapproval) and CC.
So now you tell what you want to clarify about IOD i.e., intimation of disapproval approval.
For cross verification of payment of service tax you may obtain the TIN No. or the transaction details from the builder and cross verify the same with the authorities concerned.