htop to see if there are any weird processes.Also do netstat when all the connections you initiated are closed so you know what exactly is going in and out. ss also is a great utility. what more, get a firewall up and running. I would prefer a gui if it was for me since its annoying to handle the cli at such times. but up to you. And keep a track of what services are added to the startup. Just a few common steps towards confirmation