Please define how you are using "level of consciousness".

My research is about training in hypoxia. The Research Committee suggested me to check the level of consciousness, along with the use of pulse oximeter while athletes are training for assess the brain function of athletes to prevent them from symptoms that may be caused by exercise in hypoxia. So, I want to know about method or test that can assess LOC.

You can probably found some method in these articles. Some are about exercise in hypoxia, some about consciousness and anesthesia.

In one, determination of the "time of useful consciousness" (TUC) was inability to add two-digit numbers correctly. It would be the best and easier method for you, because others methods need some devices.

http://www.ncbi.nlm.nih.gov/pubmed/3202796

http://www.ncbi.nlm.nih.gov/pubmed/486020

http://www.ncbi.nlm.nih.gov/pubmed/1252200

http://www.ncbi.nlm.nih.gov/pubmed/4257273

http://www.ncbi.nlm.nih.gov/pubmed/20277715

http://www.ncbi.nlm.nih.gov/pubmed/24754211

http://www.ncbi.nlm.nih.gov/pubmed/22261747

http://www.ncbi.nlm.nih.gov/pubmed/9393395

I think you can also found some data in apnea competition domain (a certain consciousness level is required to validate a record)

http://www.ncbi.nlm.nih.gov/pubmed/19574501

http://www.ncbi.nlm.nih.gov/pubmed/23487544

You may want to check out the effect of hypoxia on executive function, specifically shift   . While some recent research has not demonstrated an effect on speed of processing, http://www.ncbi.nlm.nih.gov/pubmed/23675496 or working memory http://www.ncbi.nlm.nih.gov/pubmed/25460539 you may want to look at the interference aspect. Stroop Color Word interference is very easy to administer while an athlete would be exercising. There are computer or card based versions. 

http://www.ncbi.nlm.nih.gov/pubmed/23675496

http://www.ncbi.nlm.nih.gov/pubmed/25460539

You would have to define 'level of consciousness' for the purpose of your study.The bispectral index monitor is a processed EEG that generates a number between 1 and 100 in anesthesia - it is more a depth of anesthesia monitor than a level of consciousness monitor, and is used to assist in avoiding awareness under anesthesia. A number between 30 and 60 is targeted for anesthesia. A number above 60 would not necessarily mean you are conscious.

An easy well validated scoring system is the Glasgow Coma Scale and requires scoring of a response to a verbal command.

You could give your subjects a cognitive task eg copying the text of book as they get progressively hypoxic, and time either how long it takes to start failing in the task (the writing goes funny or they can't read the words), or record at what saturation level on the pulse oximeter they start to fail. 

Finally, there is the SCAT 3 test currently used in rugby union to evaluate level of consciousness of a player after a head knock - this document below includes the GCS I mentioned above

http://bjsm.bmj.com/content/47/5/259.full.pdf

SCAT 3 is a good test after concussion, but I don't know if it was tested in hypoxia condition.