Originally Posted by MamaHen
Hold your fabric up-salvage to salvage, now move the front salvage piece left to right- right to left, until the fabric looks straight from side to side. If you see little waves in it, it is not straight. It takes a little practice, but it helps. I find that more than a yard or so is a bit much to hold. I also found that if I press out the middle fold, the fabric will fall straighter. Good luck.
This is a wonderful tip that is never covered! As a quilt teacher, I use this all the time....
You need to look at the BOTTOM of teh fabric as you shift the top layer left/right ... when it appears to fall straight, your threads are likely straight.
Also, check to insure that you are not angling your rotary cutter either toward OR away from the ruler. This can cause waviness in the edge, too.
Make a very narrow trim cut to remove any excess, then if you want to check, make a cut about 1/4" wide from this trim cut. oopen it out and look at the fold(s) for the dreaded v's and w's. If there are none, you're good to go. If you see some, you need to do the "hang test" again until it comes out straight.
HTH,
Odessa