How to check if MacBook has activation lock online?

I know with iPhone it is easy with online websites - but how about MacBook?

There are no reliable online resources for this. Phones or Macs.

That any Apple device is in activation lock is not reported by Apple to anyone.

See the article How to remove Activation Lock - Apple Support

Unlock on the web 

To remove Activation Lock from a device that's not in your possession, follow the steps below. If you purchased a device that's protected by Activation Lock, ask the previous owner to follow these steps. 

1. From a web browser, sign in to iCloud.com with your Apple ID.
2. Go to Find iPhone.
3. Click All Devices at the top of the screen.
4. Select the device that you want to remove from iCloud.
5. Click Erase [device]. Select Next until the device is erased. 
6. Click Remove from Account.

To begin the setup process, restart the device.

Read Activation Lock for Mac - Apple Support