I have only had pigs for 3 years now but I can always tell quite soon. The first sign, as you have mentioned, is that if she is pregnant she won't come back into standing heat three weeks later. Shortly after this time, the nipples start to change. I am not saying she will get an udder because that won't happen for a while... but by around week 5 or 6 my girls have always without fail had more noticeable nipples, this happens with gilts and proven sows, who have had several litters, alike.
The change is not dramatic, it is subtle, but when you spend time with your animals I find you notice these small changes. No sooner I see this I know for certain that there will be piglets in a cpl of months time
Many pig keepers say that in the pregnant sow a sure sign is that the vagina tip points upwards in a pregnant sow and downwards in an empty sow. I can honestly say this has never been the case with my girls, not even once but others swear by it!
You may also notice a character change in a pregnant sow, she will be calmer, some will be lazier etc it depends on the pig!