If you know where the sample was taken, and how deep it was taken, you could use USDA-Natural Resources Conservation Service (NRCS) soil survey information that includes data about sand silt and clay. available on "web soil survey" at http://websoilsurvey.sc.egov.usda.gov
. it may not be the exact sand silt and clay ratio of your particular soil sample, but unless you really need to know the exact percentages for some reason is probably a more accurate average number than what you will get by trying to test it yourself.
the finger roll method is a pretty good quick method, BTW