Recently I needed to find the selling plan ID for a new subscription product in our store. We needed it so that we could generate a short add-to-cart link which required the ID number.
However, I couldn't find any Shopify resources about how to find this number. Shopify Support suggested I use an API, but that seemed too complex for what I needed.
After tinkering around a bit, I was able to find the ID in a fairly easy/straightforward way. This will probably be pretty obvious to advanced Shopify users, but I wanted to share with other basic users like myself just in case it may be helpful.
Go to the live product page on your website
Press Ctrl + U on your keyboard or right click and select "View page source"
In the tab that opens, press Ctrl + F to open the page search function of your browser
Search for "selling_plan_id" ( or you may need to search for "selling_plan.id" ) on the page and use the number that directly follows