I created EC2 instance and security group for the RDS ( its type: PostgreSQL
and source: launch-wizard-1 (EC2 has)
). And also I created the RDS instance.
Then, Is there any way how could I check if EC2 connected with RDS correctly?
The EC2 instance has no any data, it is just created.
There are two ways to try and connect to your RDS for testing:
https://github.com/snowplow/snowplow/wiki/Setting-up-PostgreSQL
Once connected you can use your local db client like dbeaver for connecting.