Richie Lee has a method for checking whether a variable exists in Powershell:
Recently I needed to check that a variable exists in the PowerShell session currently running. This is actually far easier than it sounds. So here is a simple demo for how it works. The magic here is the “Test-Path variable:my_variable” on lines 4. It tests that a variable of that name exists. If it does, great, let’s print out the value. If not, let’s alert that it doesn’t. The second example of this on line 11 will do exactly that.
Read on to see Test-Path in action.