Google tells you the date a page was first indexed when you specify a specific date range to search within. This tool quickly appends the required variables to a google search to show the dates for any URL you look up.
Note: If a page has a date timestamp, that will usually override date of indexation. If your indexed URL changes (due to updated https or www canonical), the date of the change will be shown. There may be more instances of more recent dates being shown (especially for homepages) and I'm currently experimenting to figure out the triggers (e.g. major content updates).