A brightly colored rag or handkerchief to tie onto your car antenna. A CB radio. Just because a lake or stream is frozen does not mean that the ice is safe. Check the appearance. Estimate the strength by first testing the edges.
Snow cover can adversely affect the strength of ice.
Generally, new ice is much stronger than old ice. Several inches of new ice may support your weight. A foot or more of old, rotten ice may not. The ice's natural crystalline structure is changed by constant freezing and thawing. Ice does not freeze at a uniform rate.