you can generate traffic between the 2 sites (file transfer for example) then use show interface command to check for speed by looking to : 5 min input/output.
but you need to wait for 5 min to get accurate result. you can also change this value: under interface tunnel you type: load-interval 30 (to get the value of 30s)
if you are familiar with SNMP you can also use some free software like PRTG to measure interface speed.