When it's possible you will be crossing ice on your route, there is a couple of stages:
Planning at home
First of all, you should explore the area of your trip. The question is are the water sources frozen and how thick is the ice
- Small lakes in the forest, where there is no winds and no water flow, freeze first. If you know that a couple of small lakes in the area froze a week or two ago, then you can assume that all the other have probably frozen too.
- Big lakes, which are visible from satellites, can be checked directly on MODIS daily satellite images. After you find the lake on the image (which is difficult) you can tell if it's frozen by its color: white is frozen, dark is open water. If the lake is frozen, you should check a week or two back to see if it was already frozen and there were any chances for the ice to thicken.
Very big lakes (like, >100 km^2) may be partially frozen for months. You can see it on the images. You'll have smooth white fields near the shores (fixed ice), dark open water in the middle, and mixed dark/white pattern in between - floating ice. The trick is that fixed ice may split off and start floating any time, if it's near the edge (and sometimes far from the edge too). On the other hand, sometimes local authorities publish ice coverage maps for very big lakes.
Rivers are very treacherous. You can examine other rivers to roughly estimate that there is (or isn't) some ice on your river, but you should concider width and speed of the current. But river ice is not uniform and most rivers have areas open during the whole winter (where the current is faster and at shallow places). They may be very hard to detect, so you'd better just cross rivers and avoid walking along the river.
And give the ice some time to thicken. This depends on the temperature much. I'm not going to give any numbers here, each case is unique.
If you are going far from home, it's wise to ask locals about ice situation.
Estimating on the spot
Now you are following your route and have a (probably) frozen water in front of you. It doesn't mean you have to cross it. First, think it over again, because going back is less evil than dying or having your chilblain fingers amputated.
Check if there is any trace of open water. Avoid areas, where snow seems darker or lower. Avoid areas near water plants and rocks. If it's spring, avoid areas near shores (they unfreeze faster). Stop right away if you feel vibration or hear cracking. Never test ice strength with a kick of you foot, use a long pole instead (this applies mostly to crossing rivers, because you are not going to strike with a pole after each step on a big lake).
Unfortunately, there is no method to test that the ice is 99% safe. You are the closest when you know that a couple of people (N times as heavy as you/your party) have crossed it 5 minutes ago, but still...
Yes, you can use an ice screw to test ice thickness, but you can't do it all the time.
Alleviating the concequences of accidents
If you have fallen into the water, it's very difficult to escape yourself:
- you are cold
- you get stiffer every second
- you may panic
- ice is too slippery and/or too fragile to climb
- your clothes and equipement (backpack, skies) make it much worse
- there may be underwater currents pulling you under the impenetrable "roof" of thick ice
The main advice is
- Have another person following (but not closely!).
He will have much more chances to help you. He should have a rope ready (you too, because he might accidently fall). He should not come too close to avoid following you into the water. He should be quick. You both shouldn't panic. He will have much better success starting fire (or calling ambulance).
Other advices are:
- Drop your equipement as fast as possible (your backpack's belt should be unfastened all the time and you ski binding should allow underwater unfasteneing). Drop your heavy clothes too, it's wet anyway.
- Have a personal awl (a sharp metallic piece with a handle) very accessible (like hanging at your sholder) to climb slippery ice. Or at least use youe ski poles, etc.
- Have matches/lighter in a waterproof package close to skin, so that you can start a fire immediately after rescue.
- If it's possible, have an inflatable boat with you (I mean, inflated and tied to a roap in your hand). A warm diving suit + lifejacket + a boat make crossing not-so-frozen lakes quite easy and safe (tried it myself).
When you are out it's very important to get warm ASAP. And if the warming has to be delayed, move intensively. If you don't feel your fingers, swing your arm/leg about 50 times like a pendulum with full force. If you still can't feel the fingers, you can only save them by slowly warming it. And, yes, fingers are not as important as life.
Ice safety is very complex and this topic can't cover anything.
Take ice seriously. Really. It's deadly and it's not an exaggeration. Lots of unprepared people fall and die each winter. And I know half a ski-tour group with an experienced leader drown two years ago on a big lake. I know two sportsman lost their fingers after unsuccessfully crossing a small river. I know a very experienced and fully prepared man escaped just with a couple of deep cuts on his face - he was lucky.
If you seriously plan crossing ice in a remote area, please be prepared, find a book or big article on the subject, consult with an experienced one and use your brains.
And still it's solely your responsibility.