Ownership
The sale deed is the proof of transfer. The title chain — the run of earlier deeds — shows the ownership has never broken.
An encumbrance certificate lists loans and charges registered against the property. Ask for the last thirty years.
Permission to build and occupy
The approved plan shows what the authority allowed. Compare it against what stands; unapproved additions are your problem after you buy.
The occupancy certificate says the building is legally fit to live in. Without it, loans and resale both get harder.
Dues and registration
Property tax receipts and the society no-dues letter confirm nothing outstanding will follow you.
For anything under construction, the RERA registration number lets you check timelines and complaints on the state authority's site.