Medicinal mushrooms have been immensely popular in Asia for millennia. Today, Americans are waking up to these powerful nutraceuticals. In the wake of this new awareness follow issues of product quality claims made by competing brands.
All medicinal mushroom species are plagued by this. Most fiercely debated is red reishi (Ganoderma lucidum), the most popular of all medicinal mushrooms. So this article will primarily focus on that species, but it's generally applicable to all species of medicinal mushrooms.
First off, stay clear of cheap, mass-produced medicinal mushrooms. If they are just dried and pulverized, then the medicinal compounds are still inaccessible behind the mushrooms' cell-walls. You'll be wasting your money on inert matter.
There are many real therapeutic brands on the market, though. But even among them, there are significant differences. They all claim to be the best, naturally, because they all want to sell their particular brand. So let's sift the data, get the facts straight, and clear up misleading information that is out there.
There are primarily three ways to extract the medicinal compounds from red reishi. Each method pulls out different compounds, all of which have been proven by scientific research to be therapeutically important.
1. Water Extraction, Hot (polysaccharides and more)
2. Alcohol Extraction (triterpenoids, sterols)
3. Fermented (arabinoxylanes, etc.)
Polysaccharides have been shown to possess powerful anti-tumor qualities through enhancing the immune system and by inhibiting blood profusion through tumors. They are also strong antioxidants. 
The alcohol soluble compounds are mainly triterpenoids, a large group of related compounds. Scientific studies suggest they help stabilize cholesterol, blood pressure and clotting. Most importantly, they are the anti-inflammatory compounds so critical to many of reishis suggested uses, including arthritis, allergies and asthma. 
And then there are the new and unique medicinal compounds that get created when reishi is fermented. These "secondary metabolites" have their own therapeutic properties, different from the first two groups, including anti-tumor, immune support and blood-sugar balance. 
This article is intended to give general guidelines to help you choose an effective reishi product, not to recommend any particular brand. But there are at least two reishi companies on the market which both state only hot water extract is useful while alcohol extracts are of no value.
Of course they do that to promote their own brand. In reality both hot water extract and alcohol extract contain unique medicinal compounds that are all very important.
To determine if a brand of reishi (or Ganoderma) contains all the important medicinal compounds from the mushroom, find out if it utilizes both alcohol and hot water extraction. An additional plus would be if it also includes fermented reishi.
An additional point to keep in mind is the form the reishi comes in. Reishi that dissolves completely in a water-based liquid such as coffee probably only contains reishi from hot water extraction. As a substitute for regular coffee, this would be a superb health-choice. However, it will not contain the water-insoluble triterpenoids so valued for their anti-inflammatory properties.
On the other hand, alcohol tinctures can actually contain both water soluble polysaccharides and alcohol soluble triterpenoids. The way you know is that the polysaccharides will fall out of solution because of the alcohol, and will make the tincture cloudy. That's alright though; you just need to shake before you take. Capsules and tablets may be one or the other or both. The only way to know is ask the manufacturer or read their information.
 Boh B, Berovic M, Zhang J, Zhi-Bin L. "Ganoderma lucidum and its pharmaceutically active compounds." Biotechnol Annu Rev. 2007;13:265-301.
 Tang YJ, Zhang W, Zhong JJ. "Performance analyses of a pH-shift and DOT-shift integrated fed-batch fermentation process for the production of ganoderic acid and Ganoderma polysaccharides by medicinal mushroom Ganoderma lucidum." Bioresour Technol. 2009 Mar;100(5):1852-9.