How can I check that the file I'm trying to upload is fully successfully uploaded or not.
Generally if there is no exception your upload was successful.
Additionally, if your server supports hash verification you may compute hash of the uploaded file and compare: https://www.limilabs.com/blog/verify-file-hash-after-ftp-upload
You can use Ftp.GetSupportedHashTypes method to get hashes supported by your server:
List<FtpHashType> hashTypes = ftp.GetSupportedHashTypes();