If walking, swing a 3lb axe medium strenght, and if the axe does not goe cleanly through the ice, then proceed for a couple of steps, then chop again.
Even if the water seeps through, it is safe. Each swing of the axe will clear about 3" of ice, which is adequate for walking. If you swing the axe and chop the ice, and the axe goes thru easily, then stop, turn around and resort to plan "B" (Go home).