Is there any condition to check wether the linker consists of a "Textblock" or not?

In the screen shot when I link the linker has a textblock showing “label”…Is there any way to write a function to check wether a linker consists of a textblock or not?

If you are using the link template shown in How to get the linker selected along with the label in gojs, then you know that there will always be a TextBlock label on the Link.

It might not be visible, due to the Binding on the GraphObject.visible property. But it exists. The binding also tells you that in this case that will be true if it is visible.

In general the template provides the complete definition of both the appearance and behavior of any Part, which includes Nodes and Links.