If the address begins with 1, they are definitely not Segwit coins.
If the address begins with 3, they are held in a P2sH address. They could be segwit or could just be a multisign address. If the private keys are under your control, you should know. Likely they are a segwit address since you would definitely know if you set up a multisig address since it involves a very complicated process to set up. If the private keys are under someone else's control, like an exchange or online wallet, you will need to research or ask.
If the address begins with bc1, it is definitely a segwit address.