Subject: Random pop skill in training

How can I know that player is close to pop advance to another level in random skill training?
I mean for example that he is solid defender close to being very good...
Is it even possible to know it or it is a pure luck when randon pop appear?
Can I check it somehow in Sokker Viewer or Sokker Organizer?

Theres no way of being 100% sure, most of it is just guess work, but you could also find out how many weeks it has been since his last pop in that skill and work back, that way you would know how many general trainings that player has had. Your head coaches individual skills is what makes the difference along with the players talent. You are more likely to get a random pop in defending on a player if your head coach is skilled at magical + in that area, its less likely if he is only formidable.

Is head coach more important than his assistants?
I am training speed now and my main coach is solid and two of three assistant are excellent and one is very good...

The head is most important for both proper training and random training

Random is a poor word for it as it is exactly the same process as normal training - just about 9-11 times as slow.

The speed of these random pops is only 10x as slow if you have unearthly in the particular skill, otherwise it is even slower. This is why as players get richer they progress to getting head coaches with many high skills - it improves random pops. Improving your assistants undoubtedly has an effect too. Both these improvements are increasingly expensive