Can you tell at age 15-16 months if child has a speech impediment?
IMO, you don't worry unless the doctor has specifically said there is a delay. Many kids do not choose to talk until after two. My hub who has a very high IQ, did not talk until 3 and his first words were a sentence. It takes all kinds. Model good language and encourage all attempts your child makes.
Yes, a Speech-Language Pathologist can assess your child for a delay in their speech or language development as young as 12 months. However, most insurance companies will not cover speech therapy until 18 months as it is difficult to qualitlfy a child under 18 months for therapy based on standardized scores.
Ask your pediatrician for a referral to an SLP if your concerned. The referral will help with insurance reimbursement.
If you're interested in how an SLP would determine a speech or language delay, one typical assessment is the PLS-5 (Preschool Language Screening) that is used on children from birth to five years of age. There are several others such as the CELF-P or BDI.
Again, the answer is Yes, but you should consult with your pediatrician or an SLP.