They avoid sun where they might dry out and feed largely at night when it is cooler and more moist. To understand how slugs could possibly multiply so quickly, it helps to know that they are hermaphrodites, creatures that have both male and female body parts. What that means to the gardener is that even if there doesn't seem to be enough slugs around for much dating and mating to go on, they can manage it all by themselves when necessary. Slugs' eggs look like clumps of tiny gelatinous pearls, each about the size of a pin head. The eggs are deposited in the soil where they may hatch in a few weeks or lay dormant for months until there is enough moisture.
Slugs take about a year to mature and overwinter in the soil.
The most obvious sign, of course, would be to find the creatures. Unfortunately, that isn't always as easy as it sounds. Because of their soft bodies, slugs tend to hide out during the day when you might be out in the garden where you could notice them. They do their work at night. Taking a flashlight out at night to look is one way to check, but there are easier ways.