Most likely, the InfluxDB client and InfluxDB on the same server are the same versions. If you want to specifically check the server version, you can do this.
In versions 0.11 (at least) and higher, the influxdb executable takes a version argument:
% /usr/bin/influxd version
InfluxDB v1.2.1 (git: master 3ec60fe2649b51a85cd1db6c8937320a80a64c35)