To calculate pure upload and download speed is pretty easy, especially if you have control over both the server and the client. Just make UploadFile and DownloadFile requests (which you can do using the Invoke-WebRequest cmdlet or the HttpClient or WebClient .NET classes), measure the time it takes and divide the file size with the time measured. Do be sure to upload both small and slightly larger files, to account for connection opening and similar.
However, if the server is not yours and you keep downloading and uploading a lot of large dummy files (especially if you run it from several different machines at the same time) it could be considered less nice (bordering on DOS-attack, possibly?), in which case I suggest contacting the server owner before doing any such thing.