I usually do a break test. I take some known yarn, and break a piece with my hands. Then I take the unknown yarn and do the same thing. That usually tells me if it will work as a warp, or will be weft only.
I also take a small butterfly of the yarn and wash it by hand, scrubbing it a little. If it's going to fall apart, it will do it then.
You pretty much can't buy flat steel heddles any more unless you find them used. I don't like them either, and have removed all of them from my Macomber, replacing them with inserted eye metal heddles. You will like those much better.