There are many signs that may lead you to say, my child may need to be evaluated. Some of these signs may include, if your child is not at the same level as their peers.
So for example, if they're not playing with their friends in the playgrounds, like their friends are playing, if they are not walking, if they are not talking, if they are not engaged with you or with others, if they're not able to effectively communicate their needs and wants, if they're not recognizing familiar faces, if they seem withdrawn, if they seem that they don't want to play with other people and they just want to be left alone, any time you have an instinct that you feel, well, something seems off, or hey, you know, their friends all seem to be riding bicycles, but my child is not, it could be your child is not riding a bicycle because one, have you tried to teach them and they're not able to, if that's the case then perhaps there's a weakness there, perhaps they're not physically able to, and perhaps they do need intervention to help get them to where they need to be, which is okay. But if you have that feeling, don't delay.
The earlier you get your child evaluated, the better off they will be. So the first step is having an appointment with your child's pediatrician, and to share your concerns with your pediatrician, and to discuss whether or not your child should get a further evaluation.