How can I check if a data object exists

I have a program that is going to loop through a collection and I need to be able to check if there is info in the collection at a certain spot.

For example

Lets say I have a list of employees and I have set the key in the collection to be their ID number as a string.

Over time, employees have come and gone so employee ID # 45 may not be there anymore so how can I check if Employees.list(employeeID) has any data in it. Remember, employeeID is a string so I dont want what is in the 45th slot, I want to see if employee #45 is still in the system and if not I dont want it to crash if it is looking for employee 45. Right now I get a run time error 91 when doing so.

Thanks for the help

Can you post some of the code?