There are dozens of services that can show you the internet download and upload speeds for you. For example, there is Fast.com offered by Netflix using which you can instantly find out your internet download speeds in your web browser or through the Fast.com app. But these services do not display the data transfer rates at all the times. If you want to stay informed of how your mobile data connection or WiFi network is performing all the time, then you can make use of the free Android app called NetSpeed Indicator.
After you have installed the NetSpeed Indicator, it will instantly start displaying the internet download speed in the notification bar of the Android screen. The download speed usually stays at 0 until some app tries to access the internet. Even then not many apps fetch too much data and the speed drops back to 0. This does not mean that your internet speed is 0 or a few kbps. It just means that no app is using the internet or the apps are not using the internet at full speeds.
If you want to see how much is the maximum possible download speed is available from your data provider or WiFi network, then probably you will have to try downloading something from the internet or try playing a YouTube video. As the download is in progress, you will be able to notice the maximum download speed in the notification bar.
In the settings of the NetSpeed Indicator, you can set the data speed units, data usage units, notification style, display daily data usage, display daily WiFi usage, download or upload speeds and more. There is also an option to automatically start the app at Android boot so that you can see the download speed from the notification bar.
You can get NetSpeed Indicator app for Android from https://play.google.com/store/apps/details?id=com.nisargjhaveri.netspeed.