DBCC PAGE equivalent in Postgres
You can

CREATE EXTENSION pageinspect;


Then you can use the function

SELECT * FROM bt_page_stats('indexname' , 42);


to get some information about block 42 of the index: the number of live and dead entries, the amount of free space and the pointers to the left and right sibling (btpo_prev and btpo_next) and the type (root, intermediate or leaf).