Difficulties with producing some speech sounds accurately may reduce intelligibility of speech. In addition, more subtle difficulties in recognising specific sounds in words (phonological awareness) can lead to literacy difficulties. Relationship with speech disorders
Speech is the act of articulating sounds, and this can be impaired for all kinds of reasons – a structural problem such as cleft lip and cleft palate, a neurological problem affecting motor control of the speech apparatus dysarthria, or inability to perceive distinctions between sounds because of hearing loss. Some distortions of speech sounds, such as a lisp, are commonly seen in young children. These misarticulations should not be confused with language problems, which involve the ability to select and combine linguistic elements to express meanings, and the ability to comprehend meanings.Although speech disorders can be distinguished from language disorders, they can also co-occur. When a child fails to produce distinctions between speech sounds for no obvious reason, this is typically regarded as a language problem affecting the learning of phonological contrasts. The classification of and terminology for disorders of speech sound production is a subject of considerable debate. In practice, even for those with specialist skills, it is not always easy to distinguish between phonological disorders and other types of speech production problem. Speech sound disorder (SSD) is any problem with speech production arising from any cause.Speech sound disorders of unknown cause that are not accompanied by other language problems are a relatively common reason for young children to be referred to speech-language therapy (speech-language pathology).
Language disorders occurring with these conditions need to be assessed and children offered appropriate intervention, but a terminological distinction is made so that these cases would be diagnosed as language disorder associated with the main diagnosis being specified: e.g. "language disorder associated with autism spectrum disorder." The reasoning behind these diagnostic distinctions is discussed further by Bishop (2017). Benchmarks for children with developmental language disorder
Common red flags at one year of age:  

No reaction to sound
No babbling
Difficulty feeding
No imitation
Limited use of gesturesAt two years of age:

Makes minimal attempts to communicate with gestures or words
Has not spoken their first words
Difficulty following simple directions
Inconsistent response to "no"At three years of age:

Limited use of speech
Incomprehensible speech
Limited understanding of simple questions
Difficulty naming objects
Frustration related to communicationAt four years of age:

Uses only 3-word phrases
Speech is not understandable to parents
Takes a long time to understand others
Difficulty asking questions and finding words to express thoughtsAt five years of age:

Speaks only in simple sentences
Speech is not understandable to teachers
Difficulty answering questions
Difficulty with complex directions
Difficulty telling stories
Difficulty with peer interactions

Assessment
Assessment will usually include an interview with the childs caregiver, observation of the child in an unstructured setting, a hearing test, and standardized tests of language. There is a wide range of language assessments in English. Some are restricted for use by experts in speech-language pathology: speech and language therapists (SaLTs/SLTs) in the UK, speech-language pathologists (SLPs) in the US and Australia. A commonly used test battery for diagnosis of DLD is the Clinical Evaluation of Language Fundamentals (CELF).