How to find out what region a movie disc is?

I’m assuming all of the dvd’s I have are region 1 but how to find out for sure? Is there a program out there for free that will let me check this? I also noticed on most of my commercial dvd’s, they say region 1 or NTSC.
But some of them do not say anything regarding region, even on the dvd box. If I wanted to sell some of my dvds and needed the region info of the dvd to inform the buyer, where would I find this out?