Tinkering with an old generator showed me how to spot bad diesel fast

I picked up a beat-up diesel generator from a yard sale to fix in my free time. It kept sputtering out after running for a bit, and I was sure the injectors were clogged. Instead of pulling them right away, I tried a simple trick my dad once mentioned. I drained a small sample of fuel into a clear jar and let it sit for a minute. If you see a layer of water or specks at the bottom, the diesel is contaminated. This quick check saved me hours of work because the fuel was just dirty, not the parts. Now I always test the fuel first on any diesel engine that acts up. What's your go-to method for checking fuel quality without a full kit?