As an indication of ice strength, check its colour. Clear blue ice is strongest, white opaque or snow ice is half as strong as blue ice while opaque ice is formed by wet snow freezing on the ice. Grey ice is unsafe - the grayness indicates the presence of water.
This time of year is an environmental factor affecting ice thickness – other factors to affect ice safety include: chemicals such as salt; fluctuations in water levels; logs, rocks and docks absorbing heat from the sun as well as changing air temperature. Shock waves from vehicles travelling on the ice is another factor affecting ice safety, as this one couple discovers in this video:
Learn more about ice safety, such as what to do when you're alone or with others on ice.