For one message: in a new email, open Options and select Request a Read Receipt (you can also request a delivery receipt there). For all messages in classic Outlook for Windows: File, Options, Mail, then the Tracking section.
A receipt arrives only if the recipient’s program supports it and they agree, so silence does not mean the message was not read. If you want another signal, HawkOpen works in Outlook on the web through the extension, and from the Outlook desktop app by sending through the HawkOpen dashboard with your connected Microsoft mailbox; it tracks only recipients whose consent to tracking is recorded for your company.