I use the basting down of the batting at the top to test stitches...I don't use a mirror...what I do is run my fingernail along the stitches on the underside of the quilt...if you feel a tut, tut, tut as you go along...then it's time to unpin (I unzip) the backing at the takeup bar a little ways, and check the stitches.
Wonder how many folks get a class in using their machines? I didn't, had to teach myself. Thinking about offering some classes here (in my home) for those interested in learning about longarm quilting...from preparing to finishing the quilt on a longarm...and one on maintenence.