To ascertain the strength of the wind [...]
At about 30-40 mph moderately heavy deck furniture begins to evacuate.
At 50-60 mph the trees are swaying heavily.
Above about 70 tops of trees break and sometimes trees uproot. Dead or weak trees may snap.
I haven't tried watching blades of grass or paper fly away. :-)
We had 120 mph winds in 2005 so a lot of the weaker trees are already gone. From what I heard only 60 mph winds in the last snow storm (which produced almost no snow on Cape Cod, but a bit of wind). I was in CA but the admiral was defending the fort.
Wind on land is only a vague indication of wind on the water.
On the water use the Beaufort scale guidlines. Once used to it you can often get within 2-3 knots just looking at the water. This is handy when its 10-15 knots in the harbor and 20 or more in open water and you can see open water. Nice to have a reef or two tied in before getting hit.
Anything less than 10 knots, who cares as long as it is enough to move you. Much above that 15 knots it is a one reef wind or two reef wind or blowing like stink in which case you may or may not want to go out at all. Not very precise, but practical.