When I used to work in the Antarctic, we used a "bog chisel" to assess how strong the ice was, the bog chisel basically being a broom handle with a 4-5 cm wide (not very sharp) chisel on the end. If you could get through with one blow, the ice wasn't strong enough (or at least when not wearing a wetsuit); if you couldn't get through with 5 or 6 blows, it was strong enough for a skidoo.
If you were wearing a wetsuit, you could go on ice that you could jab the chisel through from about a foot, and see the ice flexing underneath as you moved.
At one of the other bases (Signy Is), they used to have an annual thin ice race, across the bay and back, wearing wetsuits. This involved dirty tricks, like sneaking up behind someone and kicking the ice under their feet to break it.
P.S.
don't rely on freshwater ice being the same as the saltwater ice we were on.