You can use either esxcli or vicfg-hostops to enter/exit maintenance mode.
Note that you can use vicfg-hostops --operation info to check whether the host is finished with entering maintenance mode or is still busy.
After connecting to the host or vCenter server:
You can easily test the availability of an NTP server by using ntpdate. This utility is a command line utility which performs a one-time time sync to the specified NTP server but it can also be used to query the time without setting it by using the -q parameter:
$ ntpdate -q 10.30.0.1 server 10.30.0.1, stratum 4, offset 0.001217, delay 0.04170 8 Jan 10:19:36 ntpdate: adjust time server 10.30.0.1 offset 0.001217 sec