How can I very that I am receiving the bandwidth I am paying for from my ISP on fortigate

If you have enabled on your fgt any UTM features and depending on how the firewall rules are designed/crafted. you may/will see a decease in "throughput" because of the amount of packet inspection involved.  If you want to measure the raw throughput through the fgt without any UTM or packet inspection, either disable the UTM features on the fgt or (more easier) create a separate firewall rule for a single standalone device connection with no UTM enabled then perform a speed test. 

I would also check to see/confirm the duplex/speed on the fgt's WAN connection is properly configured - from the CLI perform diag hardware deviceinfo nic <interface name> and check for duplex and errors . eg.

# diag hardware deviceinfo nic port1 Driver Name     :Fortinet NP4Lite Driver Version         :1.0.1 Admin           :up Current_HWaddr   90:6c:ac:14:1f:ed Permanent_HWaddr 90:6c:ac:14:1f:ed Status          :up Speed           :1000 Duplex          :Full Host Rx Pkts    :375782 Host Rx Bytes   :114256901 Host Tx Pkts    :179268 Host Tx Bytes   :47491358 Rx Pkts         :375796 Rx Bytes        :119664559 Tx Pkts         :179256 Tx Bytes        :46005710 rx_buffer_len   :2048 Hidden          :No cmd_in_list     : 0 promiscuous     : 1

If you have direct access to your ISP's gateway device, you may want to check the logs on it or ask to your ISP support to check the device connection for possible duplex/speed mismatch. 

And speaking from personal experience I have seen a few rare cases where internal switches "went bad" or were misconfigured, causing broadcast storms and/or retransmission of packets.  edit: the upshot was we saw nothing out of the ordinary on the fgt side nor on the ISP side that could account for the slow speed complains from people on the inside, behind the fgt until we started looking at the switch and the internal network side.