Sometimes an encounter will look great on paper (or in your mind) but when it comes time to actually play through it, it falls flat. Sometimes this is a matter of certain monsters just not working as well together as you'd planned (or perhaps working too well together). Sometimes the terrain just doesn't do what you expect it to. Perhaps things would have worked out better with an extra terrain feature, or a different arrangement of features. Perhaps the starting positions of the monsters should have been moved. Or, maybe you're not sure how certain combinations of monsters and terrain will affect the difficulty. How do you resolve these issues without turning your PCs into lab rats? Playtest them yourself, of course!
Now I'm not advocating playtesting every single encounter that you ever put together. Playtest encounters tend to run longer than real encounters since you need to keep track of everything, monsters and "PCs" alike. I generally only playtest when I have time to kill, and when an encounter is more complex than normal. Also, if you're a new DM, new to encounter-design, or simply returned from a fairly long hiatus playtesting your encounters can be a great way to see what works and what doesn't when the dice start flying.
So how does one go about playtesting? It's just as simple as it sounds. Draw up your map and place all of your monsters down (or at least the ones that the PCs will be aware of just before the fight). This is just the same as if you were running the encounter for a group. The difference, of course, is that the "group" is controlled by you as well. Using PCs that are similar to your party's characters works best, but it can also be fun to mix things up to see how other classes fare (or, if you're like me and rotate DMing duties, you can get some more in-combat practice with your own character). You can also utilize playtesting as a player if you're trying to decide between 2 different classes or builds and want to test drive how they "feel." This is how I learned that most essentials classes are not really for me, despite the fact that I appreciate what they add to the game (and in some cases was really excited to see how their mechanics worked in play).
The most important thing to keep in mind is that you need to wear 2 different hats, and keep them separate. What I mean by this is when a monster's turn comes up, play that monster as if you were DMing and have it act like it would act. Don't refrain from attacking your "favorite" PC or move into position just so you can set up for a PCs power. Likewise when it's a PC's turn have them utilize the tactics that they normally would in-game. If your group is good at focus-firing and using the best tactics, play the "party" that way! If your group has trouble operating as efficiently as it could if you were running everything, try to emulate that. On the other hand, it could be interesting to see how differently things turn out depending on how much strategy is effectively used (by either side).
The bottom line is that sometimes things work differently in play than they look on paper. Conducting a test run (if you have time), is a great way to bring some of those differences to light, in addition to experimenting with different variables (monsters, terrain, PCs, tactics, etc.).