Only buy from a reputable supplier, research that supplier.
Most turquoise available now in either Stabilized (S, resin impregnated), Dyed (D, for better color), treated (T, heat treated to bring out better color), reconstituted (R), or is imitation (I, Dyed Howlite or Magnesite).
S - Soft material will be stabilized so it is more durable and easier to work. Think Chalk Turquoise.
D - The more blue it is, the better, richer copper content. The greener it is, less desirable, higher iron content.
T - Like many other rock/gemstones/crystals, turquoise can be heat treated to make the color(s) more vibrant and richer. Sapphires, Rubies, Quartz, Agates, and others.
R - Stone dust from the various cutting operations will be saved, dyed, and then stabilized to create stones of various types. Think of a poured counter top for a kitchen.
I - Howlite and Magnesite are just two of the natural stones that will be dyed to look like turquoise. Due to the veining of these stones, when dyed it can be hard to determine the real from the fake.
The only natural colors of turquoise are Blue, Green and White. Blue and green can be found in various places around the world. White turquoise is found in only one place in the world. A single local in California. It is commonly called Buffalo Turquoise.
Many reputable websites have pictures of the most common turquoise types, colors, and locations of where it is found.
Look for what is called Old Turquoise, less chance of it being fake.
Just remember, turquoise has been processed thru stabilization, dying, and other treatments since the early 1970's. If there is a question as to the authenticity of the stone, have it tested. Tests could range from color durability, chemical analysis, chemical testing, and many others.
Good Luck and happy hunting.
Jon