LabVIEW's "Threshold 1D Array" VI (located in the Array subpalette of the Functions palette) may be helpful for you.
If you set the threshold input to 0 and the index input to the start of your array, it will basically search through the array and return the first index where the element value changes from negative to positive. Then if you call the "Reverse 1D Array" function, it could detect the positive to negative value changes from the original array.
For a detailed explanation of this VI, please see its associated help file in LabVIEW (place the VI on your block diagram, right-click, and select Help).
Hope this helps!
Sonya
