Full understanding of the developmental norms and where the child is at in regards to them
No preconceptions about the child from other settings that might sway your judgement
What one person sees as a positive, another might see as a negative so training is essential for the practitioner eg a child who carts around a basket of food all day is a. not joining in other activities fully b. following a schema
Full understanding of the child (without prejudging obviously!) so you can assess without expecting too little or too much esp important for SEN (or gifted & talented)
All I can think of! Hope it helps! Sarahx