Understanding your child's sensory difficulties is a long involved process that will require observation, interpretation and re-interpretation as the child grows and new factors influence their development.
The basics require you to understand that as well as the five sensory systems you learned at school (sight, smell, hearing, touch and taste) there are three other equally fundamental sensory systems:
A child needs to be able to first of all register an input from a particular sense, then understand what that input means, then regulate that input to the right 'volume', before effectively integrating it with all the other seven senses to be able to efficiently produce an appropriate response to the situation/ task in hand.
These eight sensory systems individually need to be at different levels of alert depending on the activity in hand and your body needs to be able to change its level of alertness seamlessly if you change the activity you are doing.
It's a bit like a studio mixing desk when the technician (child) is trying to find the right volume, treble, bass, balance and blend of instruments that's appropriate for the song (activity), only in this case the song is always changing as the child moves through all the activities they are required to do on a daily basis.
In order to understand your child's sensory preferences it's useful to look at your own - this booklet helps you do that.
Online resources to help with sensory difficulties:
Making sense of sensory behaviour
Senses workshop video