How do I find out if I’m already on a monthly icloud plan

apple charges for its services monthly and automatically so you don't have to worry about paying every month, apple will send you an email of your invoice every time your account gets charged for your iCloud plan.

if you want to find out what plan are you on. On your iPhone, iPad, or iPod touch, go to Settings > your name > iCloud> Manage Storage.

You get an email for the charge.

If you are already paying 99 cents per month for iCloud Storage, you will be on a monthly plan and Apple will not charge you twice.

If you have options to downgrade, you are already on a paid plan

Take a look here > Downgrade your iCloud storage – Apple Support