Start a quick and dirty tcp server:

sudo yum install -y nc
nc -kl <port> &

Where -k is for keep alive, -l is for listen and <port> is the port to listen on.

I used this to check the configuration of an AWS ELB using TCP:5000 to health check an EC2 instance before I installed an application.

But you might just want to use it to check a security group and / or ACL surrounding the instance:

nc <host> <port>

Where <host> is the server and <port> is the port to connect to.

The application is a JVM application that runs a discard server on port 5000 specifically for heath checking purposes.