In order to be as effective a coach as possible, you're going to have to know some things about your athletes. This isn't a comprehensive list: each athlete will be unique and will require more and different information. However, these are some questions you should be asking all the time.
1) How are you feeling? Deliberately leave this question open ended. Don't create conditions like "how are you feeling physically?" Let the athlete answer with whatever is on the top of his head. If he wants to talk about his knee hurting or how stoked he is about his new 1RM deadlift, great. On the other hand, if he wants to talk about stress from work or his recent promotion, that works too. This is probably the most important question you will ask the athlete. Pay close attention to the answer: your athlete may answer with the obvious (injuries, stress levels, etc) but if you are attentive, you may notice things that the athlete doesn't recognize for what they are, such as indicators of overtraining.
2) How's your diet? Ask him to break it down for you: what did he eat yesterday? I always follow this question with the offer of assessing a 3 day food log, if the athlete is willing to take one down.
3) How are you sleeping? Again, leave this one open to interpretation. If the athlete is sleeping poorly, he may give you reasons why, be they physical, mental, or emotional, training related, work related, or family related. If they're sleeping well, make sure to take note of the conditions they are currently living in, as well as you can: when someone performs a movement correctly, we tell them "remember how that felt, because that's how it should feel every time." Similarly, identifying factors in an athlete's life that are having a positive effect on his training can allow us to recreate those factors when things aren't going as well.
4) How do you feel about your training?
Specifically, you want to know:
1) Where does the athlete feel he is making the most progress?
2) Where does the athlete feel he is making the least progress?
3) Does the athlete feel anything is lacking from his training?
4) Does the athlete feel anything is too prevalent in his training?
5) Is the athlete reasonably comfortable handling his current workload?
Along with asking your athletes these questions, here's a big question you need to ask yourself as a coach:
What are the connections between my athletes answers?
If ten of your athletes say "I think I'm running too much," you might be programming too much running in. If ten of your athletes say "I'm not recovering, I'm irritable, and my heart rate is elevated when I wake up," you might be overloading them. If ten of your athletes say "I've really enjoyed the last few weeks, I feel like I'm making a lot of progress," you're doing something right, and should work to determine what it is so that you can recreate the effect.
Post questions you think it's vital to ask your athletes and other thoughts to comments.