Once you get the rhythm of your 1/4 inch seam down you will start recognizing when it is off. My first few blocks just didn't seem to work quite right and the woman who ended up teaching my "Intro to Quilting" class would look at them and say..."your seam is bigger than 1/4 inch". Darned if she wasn't right. She could tell in a look.
We calibrated my machine (my way of saying we figured out where to set the needle for the 1/4 inch seam) on my quilt. After finishing 30 star blocks, I can tell a 1/4 inch seam too. I can promise that once you figure it out and practice it will get easier. My blocks aren't perfect but my points now usually match. I might need to trim down slightly but that is far easier than trying to add fabric in the end. (Oh yeah, I also learned that I'm supposed to PRESS not iron. Go figure. I bought an iron :lol: )