Ola Hallengren maintenance script - how can I tell a backup file is still being written to?
You could use POWERSHELL and the COPY-ITEM cmdlet.  

It will skip over files that are currently in use.

$ErrorActionPreference = 'SilentlyContinue' prevents errors from being displayed due to locked files.

powershell.exe "$ErrorActionPreference = 'SilentlyContinue';Copy-Item 'C:\Program Files\Microsoft SQL Server\MSSQL13.MSSQLSERVER\MSSQL\Backup\*.bak' c:\TestOut