Firstly, on your FiveM main menu, click "Settings", otherwise known as the "FiveM cogwheel" shown below:


Then, try to find and enable "Streaming progress" there, shown here:


With that enabled (and whenever you are in-game) you should now be able to see the progress of downloads (custom vehicles, peds, maps, etc) in the bottom right of your screen.

This is useful for those who ask questions related to streaming (e.g "why are cars not spawning?") so you can find out if your internet connection is too slow to download streamed content on the servers.

As displayed in the option, this is not recommended for players that run the game on a hard drive and not a solid state drive.