As an instructor here's what i look for to determine if a student is ready to solo.
1 safety safety safety can the student start the plane, taxi, and control it without harming himself or others
2 control can the student execute a go around, line up on approach and not over fly the pits. can he do a
nice landing and take off (not perfect) nice
3 judgement i want the student to know when its best to go around. and to know when trouble is lerking
before he gets into it
4 i want the student to land and take off from both ends of the field, and do some cross wind work. low
and high altitude stalls.
the goal here is give the student the best possible chance to go home with an aircraft in one piece when he's on his own.
A new pilot is most likely to chrash within 10 flights of his solo. i did and so have many others