Ahhh, field trips!! Loved them as a kid, hate them as a school nurse.
So most teachers are really good about letting me know when there are field trips. Guidance sponsors a lot of field trips too, and I get info from them. I am included in the "All staff email" that goes out, and notices of field trips are sent out a week or so in advance as a courtesy to the other teachers (I work in a high school). I don't even know if there is a master schedule that lists the field trips.
The part that irritates me is they still accept permission slips the day of the field trip. The bus is leaving in 10 minutes, and I just got a list of 10 more kids that I need to get epi-pens and dialy meds ready for- I have actually held up the bus because I wouldn't let them leave until I had everything situated. They stopped taking permission slips on the day of after that.
I would request teachers inform you of field trips. It should not be your job to track this down. Teachers have a checklist of things they need to take care of before they leave school with a gaggle of kids and notifying the school nurse ahead of time
should be on that checklist.