1. Open FiveM
2. Select the server you going to play on.
3. Sometimes it will pop-up and tell you what version server is using. This is a great indication to know what to inject.
4. If the popup didn't show, this means you already on that version.
5. To find out your version, open Task Manager (CTRL+ALT+DELETE) choose Task Manager.
6. In Task Manager if you never opened it before click on More details as shown here: https://i.imgur.com/cdQf7CK.png If you already used Task Manager before, click on Details as shown here https://i.imgur.com/5WlnBuY.png while FiveM open, scroll down and you will see FiveM_bXXXX this is the version you on.
If you see only FiveM without any b_XXXX this means you need to inject FiveM (Old build)