Need assistance with if statement

Hi,

I am looking to create an If-statement which checks if a refrence is set. If you go into Tools>Refrences there are two refrences that needs to be checked before you can run the macro. The names on them are:

Tekla_Structures
Tekla_Structures_Model

Both of these refrences needs to be checked.

I wanna create some kind of If-statements that controls if these are checked. And if they are unchecked it should appear a messagebox that shows instructions on how to "check" them and ends the sub.

Would appreciate any kind of help!

/ Rinor