I'm a student of Applied Wing Chun under SiFu Duncan Leung, and he has always told people that if your not sure about your teacher ask to punch them. Don't let them tell you the pattern, but keep it random. If they can cover within the real of reality then they are most likely legit. As for sparing, it is not fighting. Sparing helps with your timing and distance, to learn how to fight on the street you unfortunately have to experience it. Also, to guys beating each other up doesn't make wither one much better. Normally you should start with a senior guy giving you pressure with only a few movements, we at first use just hands and as you confidence grows so does the spectrum of what the aggressor will use. And Wing Chun works fine against Muay Thai, but you have to train timing and distance correctly. Sorry for possible being slightly off topic.