The only way to see if a number can receive texts is by sending it a text message. If it doesn't receive a text, it may appear under the Bounced section of your delivery report.
To view this information, go to your Outbox.
Then, select that last text campaign you sent out to open up the delivery report.
In your report, select the Bounced card or click on "Detailed Report". This will open up a list of your bounced contacts.
On the right, select "Opt out these contacts" if you wish to remove them entirely and keep them from being uploaded again. Or, select "Export these contacts" if you don't wish to remove them permanently and want to instead add them to a new group list.
Important Note: Once you have opted out contacts, they will not be able to be re-added to your account unless they are able to opt in using the keyword from their device.