How to create virtual indexes on postgres

Hi,
I use Postgres 9.4 database.Now,I am optimizing the queries by using
the results of "explain" and "explain analyze",Sometimes I am creating
Indexes to optimize them. But, I was not successful sometimes as even I
create Index to optimize them, the planner is not using them .

So my question was can we know whether the planner will use the index
before actually creating a real Index..or can we create "virtual" or
"Hypothetical" Index those can only be known to the planner and not the
user or Is there any alternative to do it..If present,share with me.