Obviously it helps to know where crevasses are most likely to form and avoid them. Glacier ice moves slowly downhill by plastic flow, like very stiff treacle, and will stay together as long as it doesn't have to move suddenly, then it will crack to form crevasses. These occur typically where the glacier has to accommodate sharp bends or sudden changes in slope. Thus crevasses can radiate out from the inside of valley bends or be across the glacier as it travels over a drop, which if large enough can form an icefall. The middle of the glacier is often the safest line to take.

There are two types of glacier to consider here: dry and wet. A dry glacier has been stripped of snow to reveal the underlying ice, which makes crevasses much more obvious. A wet glacier is covered by snow so that many of the crevasses may be concealed, requiring a lot more care and use of the rope. The location of crevasses can be only be guessed at from dips or cracks in the surface. The only time I would not use a rope on a wet glacier is when travelling on skis, but even then I would rope up if the glacier was heavily crevassed or the visibility was poor.