How does someone see something when they have one eye crossed?

It probably depends on the angle of the squint and what the brain does with the information coming from both eyes. If the deviation is large enough, spatial vision can be severely impaired, and double vision can also be a symptom.

However, this isn't the same for everyone. For example, my boyfriend doesn't notice any impairment at all due to his slight squint (visible, but not so that his eye drifts completely towards his nose); he generally has no problems judging distances and, for example, has no difficulty in ball sports, where spatial vision is important.