If this is a property then you can do this by adding a breakpoint to the set method of the property. Putting the cursor in the set statement and hit F9 will create the break point.
If this is a field then there's no way to watch this directly. Breaking when a field changes a value is a supported operation in C++, known as data break points, but is not supported in the CLR. The best work around is to convert the field to a property temporarily and break on the set statement.
EDIT
Updating based on OP saying it's a 3rd party DLL.
In this case you want to use the Break at Function feature of Visual Studio. The first step is to disable Just My Code.
- Tools -> Options -> Debugger
- Uncheck "Enable Just My Code"
Next actually set the named break point
- Open up the break points window (Debugger -> Windows -> Break Points)
- Click on the new button and select "Break at function"
- Enter the name of the property. For example: Position.set_Altitude
You may need to fully qualify the name in order to get it to work