Paper Stock
Pick up the invitation and throw it. The farther it goes, the heavier the stock and the more expensive the invitation. If the invitation goes far, you can expect a top-shelf open bar and decent food; if it falls slowly to the ground, hit the ATM to pay for the cash bar and have a sandwich before you leave
Font
Calligraphy? The fancier the script, the more formal the wedding. Polish your shoes and get ready for a long night. Raised glossy type? A very big affair. Basic sans-serif type? Probably a little more casual and low-key. Wacky font? Consider staying home
The Announcement
First, are the parents a package deal, e.g., Mr. and Mrs. So-and-So? If so, they're still together; if not, they're probably divorced and there is bound to be some tension in the room. Second, is it just the bride's parents at the top, meaning they're your hosts, or is it the groom's too, meaning they're splitting the tab? Either way, it pays to know who your host is so you can thank people accordingly.
Spellings
That extra u in honour? The time and year spelled out? These people take weddings very seriously. Long ceremony. Receiving lines. The works. Bring a flask.