It won't be necessary to know the contact's phone carrier right away unless you are unsure that the phone number is a landline. Phone numbers that are landlines or invalid numbers will bounce meaning that the message will not go through. Please note that even if you attempt to send messages to a bounced number, will still require a credit for attempting to send that message.
When a phone number bounces, you will be able to see this information in your delivery report.
To find the delivery report, go to your Outbox and click on the previous message you have sent.
Once you are there, you can click on the "Bounced" section and opt out or export those contacts to see which ones you would like to remove.
Note: Opt out permanently removes those contacts and they won't be able to opt in unless they text in your keyword.