Body calipers are the cheapest and most reliable way to measure body fat %. Depending on the system you use you can take 4+ measurements from your body, Tricep, Bicep, Waist and Back, total the measurements and refer to the graph that will come with the instruction manual.
Job done. I've got scales, hand held things etc and I always come up as over weight with a % of about 24% on these. On calipers I come up about 16%, which is more accurate (at least that's what I tell myself anyway).
I had my body fat done a few years ago with calipers and was 17-22%, look almost the same now as I did then but yeah the BIA scales are always a few %s higher.