Most pet stores sell ammonia test kits. These kits check the total ammonia levels (which includes both ammonia and ammonium). The problem with this is that the tests do not distinguish between the two distinct ammonia levels, meaning you may not be able to accurately assess how toxic the water is.  As a general rule, if your tank is established (meaning it's been inhabited and has active bacterial colonies), you should not be able to detect any ammonia with a standard kit. If these tests indicate a measurable level of ammonia and you know there is already a healthy bacterial colony and a lack of organic matter, it's most likely a problem with your filter. Your tank's pH can directly affect the ammonia levels in the water. By measuring the pH levels on a regular basis, you'll help ensure that ammonia levels are not toxic.  The pH of a body of water affects how much ammonia is ionized versus how much remains un-ionized. You'll still need to treat the water beyond adjusting the pH, as acidifying the water will not actually break down the ammonia that's already present. Depending on when you test your tank's water, you might be getting an artificially-high reading. The best time to test the water would probably be right before feeding, as new food has not yet broken down in the water.  Ammonia levels reach their peak approximately 90 minutes after your fish have been fed. Testing the water shortly after your fish have eaten (and are producing waste) can give you an inaccurately-high reading of ammonia levels.

TL;DR in Vietnamese:
Mua bộ kiểm tra nồng độ amoniac. Đo độ pH trong nước. Kiểm tra nước đúng thời điểm.