How difficult are the CDL license tests? It all depends on what type of commercial vehicle you want to drive.
Different vehicles require you to pass endorsement tests.
There are several endorsement tests each is about 25 - 40 questions long at the DMV.
There are three vehicle classes A, B, and C.

1. Class A vehicles are tractor-trailers.
   Most difficult class to acquire.
   
2. Class B vehicles are usually straight trucks with a gross vehicle weight 26,001 or more pounds.
   
3. Class C vehicles are vehicles that transport 16 or more people.
   Class C vehicles include buses, school buses, and smaller vehicles carrying hazardous material.
   

There are a total of 7 written tests.
Everyone must take the general knowledge test, but the other endorsement tests are required based on the vehicle you plan on driving.
The written tests are listed from hardest to easiest.
Every endorsement costs about $5 to take.

1. Hazmat endorsement.
   This is the most difficult endorsement to acquire because of all the safety regulation on hazardous material.
   
2. Air brakes endorsement.
   Getting your head around air brakes is difficult at first, especially if you have never been around commercial vehicles.
   This endorsement is required if you are going to drive any commercial vehicle equipped with air brakes.
   
3. Combination endorsement.
   There is a lot to know about coupling the tractor and trailer, how to make a proper turn, and loading a trailer.
   This test is pretty difficult.
   
4. Passenger and school bus endorsements.
   These endorsements are tough because most buses are equipped with a dual air brake system.
   To get the school bus endorsement you also need to pass the air brake endorsement.
   There is a lot of procedures to memorizes regarding loading and unloading of students.
   
5. General knowledge test.
   This is the longest test.
   Some of the questions on this test are self explanatory and can be figured out with logical reasoning, but there is a lot of questions.
   
6. Double and triple trailer endorsement.
   If you passed the combination endorsement this test should be quite easy.
   You must understand how the air system works in order to pass this test.
   
7. Tanker endorsement.
   This test requires memorizing a little bit about baffles, bulkheads, and smooth bore tankers.
   There is not much information, but the information is new to everyone.