I do a lot of tests. I'm a prewasher anyway, but I'm "making sure" a lot more than trusting to luck. I know many people have never have a bad fabric, but in all my years I had one encounter and never want another. To be honest, my normal prewashing as I did it back then wasn't enough and I didn't catch it. I now use the dye catchers, mostly to see how much dye is running so whether or not to treat it in some fashion. I'm finding there isn't much that original Dawn dish detergent (use hand wash/sink) and/or boiling doesn't fix...
When I am doing construction methods, I'll do test blocks or a test project. Sometimes my donation projects are me warming up on skills, like last year I did a LeMoyne star with all set in seams just for practice -- it had been awhile but I feel confident in my ability again instead of our handy dandy nifty no set in versions.
Some times I do tests on how I want to construct/cut the blocks before committing. I like to do all of one step at one time at one way -- that way if I mess up I only have to figure out one fix.
When I have embellishments, I do a variety of tests. Color fastness (boil), shrinkage, and durability. Last year I used blanket stitch embroidery on applique. Had to test multiple ways but felt good about using it, that it would hold up to whatever I throw at it and I throw a lot...
I'm constantly testing products and other ways of doing things, sometimes for me that is because of my vision issues. For decades all I used for all my quilting, top, bottom, piecing and quilting was Dual Duty. I started doing more with thread about 20 years ago?? Even more when I used a long arm. Last year was a deliberate test for me using So Fine as a bobbin thread in my domestic machine for piecing, not just quilting and I found I liked it and the results -- so much so I bought an auction's worth and now have enough for the rest of my sewing life.
I just mentioned it in another thread, but something I particularly want to try is the pencil by the Pounce Pad people. Love those pounce pads and think it would be a good marking tool for me.