We will assess your child's speech and compare their development with the expected ages of acquisition for speech sounds.
We know that children usually learn to say speech sounds in a particular order. Typically they also use rule-based substitutions that we call phonological processes. For example, many children under 3 and a half years of age produce the K and G sounds with the front of the tongue instead of the back of the tongue, which means they substitute with T and D, so car would sound like "tar" and gate would sound like "date". We call this process 'fronting'.
When children follow this same sequence of development and use these typical phonological processes, but take longer to correct them, they have a speech delay. A child who is still fronting at 4 and a half years of age would be diagnosed with a speech delay.
A table outlining the ages that phonological processes are usually eliminated is found on this website by Caroline Bowen.