Well quite honestly the game knows with absolute certintay the speed of the vessel but that would not be very realistic.
If you have autotargeting on and you 'lock' a target check the position keeper in the scope or Uzo and it will tell you the targets speed.
If you have manual targeting it is quite another matter and can become a fascinating study in target motion analisys. There are a number of well written guides (see stickys at top of forum). It boils down to taking a couple of fixes measuring the time and distance between those fixes. (Pencil, ruler, stopwatch).
As to map contacts and contacts called out by your crew there are thre catagorys Roughly speaking as follows
Stationary = 0 knots ... duh
Slow= 1 to 5 knots
Medium= 5 to 10 knots
Fast=9 to 20 some knots all depends a task force typically travels at around 12=15 knots.
Very Fast= Its an aircraft get the hell down
