If you need to keep track of the number of objects :
System pick all Object1
--- Object1 is on screen > Set Global Var to Object1.PickedCount
Now Global Var is the number of Object1 on screen. Whenever Global Var is 0 then none of them are on screen.
If you don't need to keep track of the number of objects :
Object1 is on screen > Set a variable to true
This would mean whenever any Object1 is on screen then the variable would set to true, if set to false then this means no Object1 are on screen.