If you don’t know what VM and FSM is, I would suggest to read below articles:

PostgreSQL: What is Visibility Map (VM)?

PostgreSQL: What is a Free Space Map (FSM)?

PostgreSQL 9.6 introduced a pg_visibility module to get the visibility map information of your table, and it is also giving page level information.
For removing fragmentation, and improve the query performance we should check the visibility map of our table.

For that, you need to create a pg_visibility module in PostgreSQL 9.6 and use it.
Previously, people are checking the suffix _vm files which associate with your relation.

Check the below demonstration on how to use pg_visibility:

Create a pg_visibility extension:

CREATE EXTENSION pg_visibility;

Create a sample table with 10000 dummy records:

CREATE TABLE tbl_checkvmap AS SELECT generate_series(1,10000) AS id;

Check the visibility map:

SELECT * FROM pg_visibility('tbl_checkvmap'::regclass);

In this post, sharing a small note on one of the internal architecture things of PostgreSQL.

Each Table and Index has an FSM which used to keep track of available space in the relation.

It stores all free space related information in an alongside primary relation and that relation starts with the file node number plus the suffix _fsm.

If you do not know about the MVCC (Multiversion Concurrency Control), please visit below an article.

What is Multi Version Concurrency Control (MVCC)

Because of MVCC architecture, PostgreSQL generates dead tuples on each update and delete action.

Why VACUUM requires in MVCC because VACUUM removes dead row versions in tables and marks the space available for future reuse.

The VACUUM process also updates the Free Space Map and using VACUUM FULL we can recover those free spaces.

The FSM pages are organized as a tree.
The bottom level FSM pages store the free space available on each page, using one byte to represent each such page.
The upper levels aggregate information from the lower levels.

We can also examine free space pages using pg_freespace module.

CREATE EXTENSION pg_freespace;
SELECT * FROM pg_freespace('table_name');