incorrect pH is the number one cause of problems.
It may not be your problem but it should be the first thing to check when trying to solve a problem as well as thinking if you've done anything differently to normal or there's been any abnormal weather or other changes recently.
You have 3 main options for checking pH.
pH pen
pH drops
pH strips
when you water you need to collect some of the water that comes out the bottom of the pot and test it to find out what the pH is. You also need to check the pH of the water you give your plant.
Follow the link below to the article on PH. It has information on the importance of correct pH, charts which show the correct range for pH in soil and hydro (they require different pH levels), Instructions (including video) on how to check pH, how to adjust pH and the different method of checking pH that I mentioned above.
Check out the article as its well written and covers all the basics then come back to this thread with any more questions you might have and we can go from there.
We'll get to the bottom of it, we just need to take a systematic approach and slowly eliminate possibilities.