The common assumption is that a water main break will result in a complete loss of water, but water will typically continue to flow in most instances. Instead of a complete loss of water, most water main breaks result in low pressure. If you turn on a single fixture, you might not notice a problem. Turn on several, however, and you may notice low pressure or even extremely slow water flow.
In addition to signs inside your home, a water main break may show signs outside your home as well. While water gushing up out of the ground is certainly the most visually spectacular, it doesn't always happen. More commonly, you should expect to see water flowing across roads, sidewalks, or grassy areas. Check with your neighbors to see if they are having water issues as well, because a water main break will likely affect an entire street or even a neighborhood.