How to test if input.Lookupfield is blank / null?

How to test if input.Lookupfield is blank / null?

I am trying to understand how to best determine whether the current user input value for a dropdown lookup field is currently null through deluge i.e. displayed or selected as default "-Select-" on web app or "Select option" on mobile app. 

Currently I have the following code which works on the web site where the if formula is presenting code from calculating a Due_Date if all of the necessary input variables are not yet selected by the user:

if ((input.Service  !=  "-Select-")  &&  (input.Priority  !=  "-Select-"))
{
    Turnaround_days  =  Service_Priority  [(Priority = input.Priority && Service = input.Service)];
    input.Due_Date = input.Raised_Date.addBusinessday(Turnaround_days.Turnaround);
}

However on the mobile app, the null field values presented as "Select option" and therefore the script errors when all of the required input variables have not yet been entered by the user. 

I have tried testing if ((input.Service  !=  null)  &&  (input.Priority  !=  null)) and using OR rules against both "-Select-" & "Select option" without success. 

If you can guide me on the best way to test whether the current input on a lookup field is the default / null value as occurs on form load it would be appreciated.