Take a look at the transom and you may find a plate which has details of the boat (a bit like the plate in the engine compartment of a car). This should have the class of boat on it. This is the official guide but as people on this forum will tell you it is only a very rough guide.
The categories are (simplified version):
A - Ocean. Extended voyages where wind conditions may exceed force 8 and significant wave heights up to 4m.
B - Offshore. Offshore voyages, wind conditions up to force 8, significant wave heights up to 4m.
C - Inshore. Coastal waters, large bays, estuaries, lakes and rivers, wind conditions up to and including force 6, wave heights up to and including 2m.
D - Sheltered Waters. Small lakes, rivers and canals, wind conditions up to and including force 4, significant wave heights up to 0.5m.
Bear in mind that my Quicksilver 3.4m SIB is certified as category C, I can tell you that it would be a bit hairy, to say the least, in a force 6!
Take a look here for exactly what the conditions are at the various wind speeds: http://www.seathree.demon.co.uk/seastate/index.htm
However it has been said that a good RIB or SIB can take far more punishment than the crew can!
Best of luck and keep on posting.