I’m as guilty of doing this as anyone else:
That little asterix, usually accompanied by some little text. If you’re real lucky that little text appears at the top of the form, if you’re medium lucky it’s at the bottom but it is there. Unlucky users will get no text.
Why is this important?
It’s just one of those esoteric symbols that we that work in the tech/design world take for granted. “They’ll know that fields required, it’s got a red asterix” and yeah, potentially for the relatively internet savvy that is true.
Unfortunately the world is not full of the relatively internet savvy and that little red asterix has got to the point of an afterthought nowadays. We add it as a check to ourselves, “yep that form is clear and simple now” but for so many people it’s really not. As anyone who has wrestled with bad forms before knows It adds friction to a journey and can be a conversion killer. Couple it with non-existent live in-line validation & the user may not know until the end of the process that they missed a required field. As much as we may slave over that form, no one wants to fill in forms; they are boring, they are a means to an end and your asterix may be stopping me from getting to that end, or buying stuff, whatever your goal is.
This is a simple one, just write (required) after the field label.
If you want you can set it in small text, you can italicise it, you can colour it red.
Whatever. It doesn’t matter (actually, all of those things will probably help the user notice it). The easiest way to let a new visitor know that that field is required is to simply tell them with a word.