Q:

How do I tell if a glacier is safe for crossing?

Sure you have to look out for crevasses which is nicely explained here: How to detect a crevasse on a glacier.
But what else does one have to consider when trying to cross a glacier? 
Some glaciers are recommended to cross depending on "the conditions". What are safety related glacier conditions to be considered when you have the goal of crossing one?

A:

I'm not an alpinist, but I think that the answer relating to "conditions" is entirely dependent on the local conditions for the position on the glacier where you intend to cross.
Sure, crevasses are a big threat and should always be treated with caution. However, there are other also serious considerations, many of which boil down to ice conditions in the long run:

Ice conditions - is the ice bare, snow covered, etc? How will this affect your rate of travel and mode of travel and how you could avert a disaster (self arrest on hard ice??).
Ice conditions - where are you crossing? On the middle of a flat section or are you intending to go through an ice-fall (e.g. Khumbu icefall, one of the more deadly sites on Everest). Are there seracs around? 
Ice conditions - What time of day are you crossing? Is it sunny, hot, cold etc? How does this affect the ice and the surface? If it is (or has been) sunny and warm, the ice bridges over crevasses are more likely to be soft and collapse. Seracs are more likely to fall.
Surrounding conditions - what are the hills like on either side? Is this a run-off area for avalanches? Refer to 1 and 3 above...
Weather - what is the weather like? Glaciers tend to be in exposed areas, and can have things like katabatic winds, which could make travel very difficult, if not impossible. Finding or creating shelter in the middle of a glacier might not be easy. Refer to 1,2,3 above.

