I suggest trying with the least expensive options first, such as the last recommendation by Rob.
Windows* does not show directly if you are connected to 2.4 or 5 GHz. However, you can use inSSIDer* 3 still available out there.
Another option is to determine it by identifying your current transfer rate compared to the channel width used for each band. In this case your transfer rate is 144 Mbps which matches the default channel width (20MHz) for 2.4 GHz, unless you have the both bands configured at 20 MHz but by default 5 GHz uses Auto (40 MHz) if supported by the router as well, and a 40 MHz channel width should give you 300 Mbps. It is a bit technical but you can use the software mentioned above if you prefer.
We also need to consider the following steps so we can make sure the connection is optimized:
- Intel WiFi Products — Data rate will not exceed 54 Mbps when WEP or TKIP encryption is configured
- Intel® Wi-Fi Products — Possible interference by other wireless devices may impact 802.11n performance
- Intel® Wi-Fi Products — TechNote: Access Point interoperability issue with uAPSD
- Intel® Wi-Fi Products — Access Point (AP) firmware updates may resolve wireless network issues
- Wireless Networking — How to configure 802.11n channel bonding on Intel Wireless adapters
- Intel® Wi-Fi Products — Why am I not able to connect at 300 Mbps with my 802.11n device?