Im in a lot of pig groups. Here, facebook, everywhere I find pig clubs and groups. Quite often I see posts from people who are new to breeding pigs asking "is my pig pregnant?" So I will try to help with this post here.
There are devices you can buy that beep if your pig is pregnant, they are roughly 250 bucks and way up from there price wise and all they do is tell you if it detects fluid in the uterine wall. If you hold it in the wrong place it will only detect bladder fluid and you can get a false positive.
There are pee sticks you can purchase for pigs goats and all sorts of farm animals too. I haven't tried these but I heard they work pretty well.
Besides actually waiting for the pigs standing heat and watching the breeding happen with your own eyes it can be difficult to know for sure. You can watch your sows vulva everyday for 21 to 28 days to know if she hits another eustrus cycle after that. If she doesn't get a swollen vulva 28 days after the breeding then yes she is pregnant.
I also recommend if you are new to pigs to take photos of your pigs vulva every day for at least two heat cycles so you can track her cycles as well as learn her particular swelling at each point during her eustrus. Every sow is different just like people. Some sows barely swell at all but have a bloody discharge, some have no discharge and swell up so much you worry about prolapse. Some sows have eustrus every 22 days and some have eustrus cycles every 28 days by the stop watch. Getting to know each of your sows cycles makes everything easier when it comes to breeding and knowing if they are pregnant and when they will farrow.
Also if you have mini pigs of any kind, it doesn't help to see if they are pregnant from the side view. As most mini pigs have round bellies anyway. Your best view is from above and from behind. This rule also applies to KuneKune pigs, American Guinea Hogs, Juliana pigs, and pot belly pigs. Here is a rearview of two of my girls days before they farrowed with litters. You'll notice their milk bags are bagged out like a dairy goat.