I forget the exact calculations involved (it's been years and I'm no longer involved with that area of operations), but you want to make sure your frequency matches the grid you're connecting with to within a certain amount. I think the stations which connect neighboring areas usually have extra monitoring equipment for a variety of energy accounting reasons, but also to measure the frequency on each side to see if it's a go or no go. I believe they use synchroscopes (https://en.wikipedia.org/wiki/Synchroscope) to do this.

Edit: it looks like y'all started talking about this below and I missed it, but the information is good, so I'm leaving it up.