Clear blue or blue black colored ice free of snow, air bubbles or debris is the strongest. Ice that is white, brittle, mixed with snow, or filled with air bubbles isn't as strong and won't hold as much weight. Ice is actually very complex. Factors including water currents and depth, snow cover, and temperature all effect how strong ice is. It takes two to three weeks of temperatures below freezing to make deep lake ice capable of holding up a large amount of weight.
New ice is stronger than old ice.
If you're not familiar with an area talk with local officials, bait shops, or check with the local police department on ice thickness. Of course looking from the surface, you won't be able to gauge how thick the ice is. If the ice is untested and there is no observation station the only way you can discern how thick the ice is by cutting a hole in it. Making a few test holes as you venture further away from shore can help assure your personal safety. Ice under three inches can break any time under the weight of an average person.