monthly weight vs. actual weigth gain?
 

How can I know any weight gain is just regular monthly changes vs. actual weight gain?

I weigh every day and keep a spreadsheet of my weights. That way I can watch how my weight changes every day of the month. Then I calculate my average weight for the month based on that. If it's significantly more than the previous month, I'm gaining weight.

Yes I weigh daily or almost daily as well. Typically I see the scale tick up a bit around ovulation then it swings back down. I also see it increase on Mondays after my usual indulgent weekends. As long as by Friday I'm back into the low 140s I continue on, if I'm still above 143 then I tighten up on my indulgences on the weekend until I'm back into the low 140s. This seems to be working and so far I actually haven't had to pull back yet on the weekends. I'm not sure if this is good strategy, but it seems to be working for me.