It isn't always easy to tell if a child has a speech or language difficulty. If you're concerned and your child is not yet at school, contact your health visitor for advice.
If your child goes to nursery or preschool, talk to staff there – they will know how your child's speech is compared to others the same age. If your child is already at school, talk to their teacher or the school nurse. Any of these people can help you get a referral to speech and language therapy if necessary.
Most importantly, if your instinct tells you there is a problem, don't leave it. It can take a while to get "into the system" so the earlier you seek advice, the better.