Of course you can. At least a pretty close approximation.
Shoot a well zeroed bullet of known BC at 300-500 (or more) yards noting accurately your drop at those ranges.
Finding accurate BC's can be difficult but the G7 numbers from Bryan Litz for Berger bullets are regarded as being very accurate. Bryan Litz's numbers for other bullets are very accurate as well and can be found in his book.
Plug accurate BC's and environmental conditions (altitude, barometric pressure and so forth) into a good exterior ballistics program such as JBM.
http://www.jbmballistics.com/cgi-bin/jbmtraj-5.1.cgi
Then adjust the speed to where the program drops match your actual drops and you have a pretty accurate measure of speed for your rifle at your location.
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