#1: Materials Used
The first thing to look for in quality towels is the type of material they are made from. The best towels are made of either cotton or bamboo. Cotton comes in several quality levels:
100% Cotton - 100% cotton towels are the minimum in quality you should look for. Many durable bath towels are made of ordinary cotton.
Prima Cotton - This type of cotton is made from the same plants that make the best Egyptian cotton, but are grown in the United States. A brand name for prima cotton is Supima cotton.
Organic Cotton - This type of cotton is about giving you a more natural product. Towels marked as made from 100% certified organic cotton are made from fibers that were never treated with chemicals while growing.
Turkish Cotton - Made from cotton that's grown in the Aegean region. Turkish toweling is almost as absorbent as Egyptian cotton, and is usually fluffy and thick.
Egyptian Cotton - The highest quality cotton available. The fibers are extra-long, highly absorbent, and very durable.
There are also micro fiber towels whose primary advantage is they dry quickly after use.
Be sure to read fabric labels carefully. Look for "100%" (i.e., "100% prima cotton"). Towels labeled "made with" (i.e. "made with prima cotton") include other fibers - usually synthetics.
In addition to the type of material used, consider the fabric weight. Sheets are given a thread count, but towels are measured by grams per square meter, or GSM. A lower GSM means the towels are thinner and lighter; a higher GSM means they are thicker and heavier. I recommend only considering towels 400 GSM or higher.
400-600 GSM towels are often used for beach towels or guest towels that aren't often used. They are medium weight, and each additional 100 GSM makes the towels a little more absorbent and heavy.
600-900 GSM towels are of the highest quality. They are heavier, more absorbent, and more durable.
You may also see references to "twist" - or the number of twists per inch made with the yarn during constructing. A lower number means the towel is softer and more plush; a higher number means the towel is more durable and heavy.
Some other construction methods are of note, too. For example, if the towel is combed cotton, the material is literally combed so that only the strongest and longest threads remain. Terry cloth towels have extra yarn and longer thread loops, making them more absorbent. Ringspin cotton is made from finer, smoother yarn, resulting in a softer towel, while two-ply towels are made with double the amount of yarn, making the towel absorbent, durable, and heavy.
#3. Size Matters
It's not true that all bath towels are of the same size. Some manufacturer's cut corners by making them smaller - and some more luxurious bath towels are considerably larger. The standard size of a bath towel is anywhere from 27 x 52 inches to 30 x 58 inches. If you want over-sized towels, look for "bath sheets," which are usually about 35 x 60 inches to 40 x 70 inches.
Hand towels are 16 x 28 inches to 18 x 30 inches in size, finger towels are about 11x18 inches, and wash clothes are about 13 x 13 inches.
#4. Making the Purchase
It may seem that buying towels in person is the best way to go. After all, if you can handle the towels, you can tell by feel how soft they are, and you can look closely to see how well made they are. Unfortunately, it's not uncommon for manufacturers to apply a finish to the towels to make them feel softer or look shinier - and that finish will go away the first time the towels are washed. And unless you can see that the towels are raveling in the store (highly unlikely), it's pretty tough to tell much about the quality of the construction by just looking.
Instead, I recommend buying towels online - or at least consulting the store's website to look at online reviews. Begin by seeking out towels with 4 and 5 star ratings. From there, look at the worst reviews for the towels. Read those reviews carefully. Is the customer really complaining about the quality of the towels, or something else? It's also important to note how many great reviews there are vs. how many bad reviews there are. If, for example, a set of towels has one hundred 4 star reviews and 2 bad reviews, it's likely you will be pleased with the towels. Another thing to look for, however, is how long the customers have had the towels. Some people leave a review immediately after buying the product - perhaps even before using the product. Such reviews aren't very helpful. But reviews written by customers who've used the product for, say, a month or more, are highly useful.