When you first create a new table with an index on it and insert data into it, the size of the index will grow almost linearly with the size of the table. When you have bloated indexes, that proportion will be very different. It's not unusual for a bloated index to be significantly larger than the actual data in the table. Accordingly, the first way you can monitor how bloated an index is, is by watching the index size relative to the table size, which is easy to check with the following query:
SELECT nspname,relname, round(100 * pg_relation_size(indexrelid) / pg_relation_size(indrelid)) / 100 AS index_ratio, pg_size_pretty(pg_relation_size(indexrelid)) AS index_size, pg_size_pretty(pg_relation_size(indrelid)) AS table_size ...
