There is no way to tell if the fabric is 'on grain' unless it is torn. Tearing always goes along one thread.... all the way across. You can cut all you want but the 'one' thread won't happen. And you can't tell if that 'one' thread lines up when you have cut lines going across lots of different threads. Why would anyone be afraid of tearing their fabric? It's a good thing if having 'true grain' is the desire.
Prints aren't always printed 'on grain'. So, even if your fabric is truely 'on grain', the print might not be. I really suspect the grain if the print is off because that usually means that the fabric is stretched crooked when it goes through the printing process.
So, 'true grain' is super important when you are making clothes because you want the skirt to be even, etc., but when I cut small pieces for quilts, it's not that important. You will have lots of small pieces with the threads going all directions, especially with paper piecing, etc. I'll watch to see what others think about grain..... when I taught sewing years ago I always had students get their fabric in grain..... for clothes..... but I don't think it's important for pieced tops.