You need to make sure you’re tracking the variable in a common place that you can access. I would suggest creating a new Blueprint (based on Actor) - call it Win Conditions Manager. Drag it into the scene. Create a reference to it somewhere you can easily get to (such as Player Controller). Then call a Custom Event on that actor to handle checking your win conditions:
This is overkill for a simple problem, but overtime you are likely to evolve other win conditions (such as points, achievements, etc) and it will be easier to implement all of them in a dedicated blueprint.