As far as I know, the LTE icon on the iPhone will not show the plus sign next to it, if the phone is using the advanced version of LTE. In that case you would have to use an app to measure the connection speed to get more details.
And, of course, LTE advanced has to be included in your carrier contract.
Data plan required. LTE Advanced, LTE, VoLTE, and Wi-Fi calling are available in select markets and through select carriers. Speeds are based on theoretical throughput and vary based on site conditions and carrier. For details on LTE support, contact your carrier and see www.apple.com/iphone/LTE.
copied from: http://www.apple.com/iphone-6s/technology/