Restaurants serve many purposes for the diner. A social setting away from home, the luxury of refined service, saving time by not having to cook or clean, and possibly enjoying food and drink which the diners themselves could not duplicate from home. As someone who began his career in the restaurant business both serving tables and cooking the food, I can certainly point out the difference between a good restaurant and a bad one.
The first and most important item I look for is the menu selection. I look for variety, originality and quality. For example, if a restaurant has only a couple of salads with the same greens and only the dressing and a few vegetables to distinguish them from one another, I tend to look elsewhere. I want a caprese salad with an olive tapenade, or a toasted goat cheese salad over spring greens with a champagne vinnagrette.
I have a soft spot for fresh seafood, so finding items like sea bass and ahi tuna tend to lure me in. Selections such as orange roughy and fried shrimp tend to mean that the fish is frozen prior to cooking, and be sure to steer clear of imitation anything. You wouldn't buckle your child in a carseat with an imitation seat belt or send them to an imitation college, right? So show your palette some courtesy and make sure the crab has a shell.
When looking for a good steak I always check for prime meat. You don't want to pay $30 for a filet and have to spend the evening chewing instead of talking to your company. The meat grades are as follows: select (chewy and fit for only long, slow cooked dishes), choice (acceptable in some forms, such as a ribeye due to its fat content), and prime (the only option when ordering filet mignon, strip steak, etc.) As a rule, the better grade the meat is, the more fat it has. I'm not referring to the edge of the meat, but rather the "marbleized" quality it attains from good breeding. The white specks in an uncooked piece of meat are fat cells that melt into the meat when cooked, adding flavor. The more fat, the more flavor.
When analyzing the menu, the last thing I look for is accompaniments, including side dishes, appetizers, sauces and dressings. I certainly enjoy creative side dishes, but some restaurants can overcharge for them, so be careful you don't end up ordering a ten dollar baked potato, unless that's what you're into. Instead, look for entrees that come with an enticing side dish, like a filet with roasted red pepper mashed potatoes, or pan seared sea bass with a potato pancake and tarragon beurre blanc.
The second most important quality of a restaurant is cleanliness. Not first, mind you, because you'll probably never see the kitchen and the health inspector should have that angle covered, anyway. However, you do want to make sure the bathrooms are clean. If the bathrooms are presentable, chances are the kitchen is too. Be mindful of the floors as well, you don't want to step in a pile of french fries left by the 3 year old who sat at the table before you.
Finally, evaluate the service staff. Was the host/hostess courteous? Were you seated in a prompt fashion? If you had to wait to be seated, was the estimated time accurate? Was there a comfortable place for you to wait for your table? Did the server tell you the specials? Was he/she informed about the menu or wine list? Was he/she attentive? Did he/she replace your silverware between courses, keep your wine glass/water glass/coffee cup at least half full? Was your next course delivered only after the previous was cleared from your table? Were you offered desert or rushed out to make room?
Obviously, your personal expectations may be far different than mine, but regardless of your service standards, make sure the staff lives up to them. Remember that you are paying for the meal, and the customer IS always right!
I hope I have been of assistance in helping you to be able to distinguish a quality restaurant from a mediocre one. There is, however, only so much you can tell from peeking in a window and reading a menu taped to a door. Ultimately, you must take the risk and dine, then it's up to you. The best restaurants are the ones that YOU enjoy. Happy eating.