Moisture requirements are a huge variable and sometimes gets down to "how long can the roots go without it?"  

Water in soil does two things: keeps the root systems cool and temperature ranges narrow plus, of course, interacts with the soil to set up the chemistry to keep the plant growing and propagating.  Slow growing plants can get by with very little and even do without for short periods of time ("drought tolerant"). Others need a constant supply (usually tropicals and definitely wetland plants).

And plant requirements vary with time of year. In periods of very active growth, they need much more than when they are going dormant for the winter.

Few plant roots can handle standing water - it pushes out the oxygen that's part of the process of growing - and so you often hear about "well-drained" (read, sandy) soils that may have good organic content (which is moisture retentive).

A good rule of thumb is that most plants will be happy if the soil is damp but not wet. It will feel cool to the touch, but not stick together in a ball.  During the growing season, a lot of plants do well with an inch of rain a per week, so a thorough watering during dry spells is a good idea. You can keep soil a bit more moist by using mulches, which prevent transpiration of water into the air.   Drooping leaves also indicate the plant is thirsty because there's not enough water in the soil to pull up into the leaves to transpire in the heat of the day.  Frequent light watering sessions can be counterproductive; it will encourage roots to hang out near the surface, only to get blasted with hot, dry weather. Periodic deep watering encourages roots to go deeper, making healthier plants.

While labels and the folks at the garden center can point you in a good direction, sometimes you just have to experiment.  Soils vary a lot from garden to garden and the exact amount of sun and shade can make a big difference.  Be observant and, with time, you'll gain expertise.

Really helpful is a soil moisture meter, inexpensive and available at any garden center.  The long probe will give you an idea of just how dry it is a few inches below the surface and whether you need to haul out the hose or watering can.

good luck!