If you aren’t math inclined (or on a bike with multiple gears), a computer with cadence is the easiest way. On a fixed gear it is pretty easy to work out your cadence from your speed since they are entirely dependent on each other. After a while you just know what a given RPM feels like, though. I rarely look at the cadence field on my computer anymore.
I am glad that the spreadsheet was useful, peedm. Excel is great as long as you know what it is for (or, more importantly, what it isn’t for).