1) Try setting a cell phone or computer for "dictation" or "voice commands" and see whether a computer can understand your spoken speech.
2) This website and/or app is just _excellent_ for learning the sounds of English:
3) When anybody speaks a foreign language, the natural tendency is to hear it with your native years and think you are hearing the phonemes of your native language. Instead of hearing the real sound, you think you've heard the a similar sound in your own language. Instead of making the real sound a native speaker makes, you make the closest sound you have in your native language.
In some cases, it is _very_ hard for a speaker to hear the differences between phonemes in a foreign language.
Some mistakes are more of a problem than others. Some mistakes just result in "a charming accent." Others mean that your listener actually hears a different word from the one you meant to say.
You just have to get a language companion, or hire a teacher, who is willing to point out the worst issues to you. Very often there are just a small number of bad problems, and if you can correct them, you will improve the ability of others to understand you.
They are often hard to correct because you often cannot hear the difference at first, so the first step is ear training.
I personally am not helped much by diagrams or descriptions of tongue and jaw positions, but some people are.
I don't know much about Chinese or phonology, but some Chinese speakers famously have difficulty hearing and reproducing the difference between the English sounds of "R" and "L." English speakers actually get the impression that they reverse them. They do not. Instead they use the same in-between sound for all of them. This causes huge problems in understanding words like "world" and "girl."