Sometimes(well, rarely) the pics of the machine will be good enough that you can look closely at the front of the machine, especially the needle area. If the needle looks to be in the left position then I look closer at the knobs and/or levers to see if I can figure out what settings are dialed in. Straight stitch setting, no L-C-R, needle to the left, Bingo! Or look to see if there's a straight stitch needle plate with the accessories. That plate is a definite tell of a left needle machine.
I have a couple of them, but one I love to use is my Underwood 142b. Just not for quilting.
Cari