Check the Local Weather Forecast:

The greater the wind speed & fetch (unobstructed open water), the higher the waves.  Fully developed waves (referred to as "Sea"), can vary in height from less than a metre in light winds, to more than 10 metres in a storm!

Marine Weather Forecasts issued by Environment Canada, include predictions for offshore wave heights, forecast trough to crest.  Wave heights can vary considerably due to shoreline effects.

Looking at the water from shore, it doesn't appear rough at all!  (You can only see the white-caps of waves when they are coming towards you).  Sheltered from the wind by the land and trees, you can't gain any appreciation of how windy it is, until you get out there!  Inflatable rafts, water toys, paddleboats & small dinghy's are easily taken offshore, but impossible to paddle back when the wind is blowing hard (click on the link to read the harrowing story that two teens from Collingwood share)

Weather - Wind Forecast Descriptions;
Pleasure Craft Operators must know these terms & expressions used by and defined by the Meteorological Service of Canada (Environment Canada)
Wind speed is given in knots;
1 knot = 1 nautical mile per hour ( 1/7 longer that a statute mile)
1 knot = just under 2 kilometre's per hour

Always check the local weather forecast information before making the decision to head out, so as not to put your craft and persons onboard at risk.
Pleasure Craft Operators can obtain weather information from several sources;

• personal observations,
  
• local radio stations,
  
• local TV stations,
  
• local marine radio broadcasts,
  
• Internet, Meteorological Service of Canada (Environment Canada)
  

• Light winds:    winds expected to be less than 12 kts.
  
• Moderate winds:         winds expected to range from 12 to 19 kts.
  
• Squall Warning:  sudden, (but brief, large increase in wind speed).
  
• Strong winds:   gusting (occasionally) at speeds of 20 to 33 kts.
  
• Strong Wind Warning:     steady strong winds of 20 to 33 kts.  (prior to 2008, this was termed, "Small Craft Warning"
  
• Gale warning:  expect wind speeds in the range of 34 to 47 kts.
  
• Storm Warning:  expect wind speeds in the range of 48 to 63 kts.
  
• Hurricane Warning:         expect winds to exceed 63 knots.
  

Keep your eye on the sky - thunderstorm clouds over the water can produce sudden powerful downbursts under them.  Winds of 50+ knots are possible.  Before it strikes, secure things on deck.  Reef (shorten) sails.  Never try to sail ahead of a storms rain advance; veer off to avoid it.
Many boats carry a barometer onboard to measure atmospheric pressure.  A significant drop or rise in pressure is a useful predictor of weather change.  Low pressure often precedes a storm, high pressure generally means fair weather.
MArine FORecasts broadcast continuously on VHF Marine Radio include plain language predictions and MAFOR Codes.  MAFOR codes (when deciphered) provide detailed information, including timeframes.  Nic DiGravio provides a detailed explanation in his article published in Probassfishing.ca    (not a test subject)

Sketches courtesy of U.S. Coast Guard, Metlife Safety Tips.