Via Powershell
Get-Command exsetup.exe | foreach {$_.FileVersionInfo}Via Exchange Management Shell
Get-ExchangeServer | select Name, AdminDisplayVersion
Get-ExchangeServer | fl name,AdminDisplayVersion
Get-ExchangeServerBuildNumbers
The AdminDisplayVersion value shows the Exchange version in the format : “MajorVersion.ServicePack.CumulativeUpdate/UpdateRollup.MinorVersion”
Via webbrowser (if maps is enabled) :
https://<serverFQDN>/mapi/emsmdb/
For Office 365 use https://outlook.office.com/mapi/emsmdb/
Now you can compare the version number in this Microsoft article and find the Exchange server build :
https://technet.microsoft.com/en-gb/library/hh135098(v=exchg.150).aspx