You can get root checker app off google play. If you see superuser app on your phone, that indicates you're rooted, but still get an app to check, as it's not always the case, even if su is there.
If you already got the custom rom you can pull the battery, power on & press the vol- button when you see the blue LED, you'll get into recovery.
From there you can flash rom. Some roms are in a zip file so you need to choose zip file from card. Or if you have backup (nandroid) you go in backups & restore.
As long as you can get into recovery you're safe.
Custom roms are rooted, if you went back to stock then you need to root. Search for root with super one click (PC) or Framaroot app (root from phone).