You don't. But you also don't SNEAK around following her. Either tell her you will be there, and go, or don't go. Same with the mall.
And if you go to watch your friend's daughter, then do that. Don't go walking around trying to spy on your daughter every second.
Now, that doesn't mean that you can't (or shouldn't) check up on her. But let her know you will be doing so. Set designated times (pretty easy to do at a football game, right? start of the 2nd quarter, halftime, whatever) for her to check in with you (you don't even have to get up and look for her, let her come to you).
I'm going to assume she has a cell phone. Text her where your seats are. Set up a plan for meeting up after the end of the game.
It isn't that hard. It's high school football at your local stadium. Do you think she is planning to sneak out of the stadium and go somewhere else or something?
The reason she is so upset is probably the way you have handled things in the past.
My 16 year old son rode with our family to a local football game a few weeks ago. (sister was marching with the band as an 8th grader for 8th grade night) Once we entered the stadium, he met up with friends and hung out with them the whole time. We just texted back and forth a few times so he'd know where we were and where to meet when it was time to leave, that sort of thing. Once when we saw something we thought he'd be interested in that he might not have seen from where he was.
No big deal. But then, I haven't secretly followed him with friends at the mall, either.
Be up front with her. Treat her with respect. How would you have felt if your mom had done that to you?
There is absolutely nothing wrong with trying to look out for her and help her be safe. However, she is growing up, and you need to start teaching her how to do that for herself. Give her the proper tools, don't carry them around for her (if that makes sense). And be up front with her. If you plan to be watching her, tell her. Don't hide. Why hide? Do you think she is doing something wrong and you want to catch her in the act? That's how that comes across.