1) Clear Consistent Code formatting. If they cannot be bothered to use tab, shift-tab, and follow a style guide, it is unlikely they will make code that interacts well with what anyone else writes -- NOR is it likely that anything they make will be easily maintainable by others.
2) Ability to do simple tasks WITHOUT a framework. This is particularly true in HTML, CSS, JavaScript and PHP where the 'frameworks' are more often than not pointless code bloat, defeat the purpose of the underlying language, or try to shoe-horn into the code programming "paradigms" that don't match the language functionality. (such as the sheer idiocy of forcing the event driven MVC concept on the linear executing PHP).
You hand them something simple -- Let's use JavaScript as an example and tell them "we have <div id="currentTime"></div>, create a 1 second timeout to plug the current time into it." -- if they start out writing $('#currentTime') it's time to show them the door.
3) Ability to articulate ideas and document their work. There is nothing worse than someone with piss poor communication skills who cannot clearly state what it is they are doing, why they are doing it, in a manner others can understand it.
This last one is a skill sadly missing in a LOT of workplaces as more and more developers become reliant upon software to do things like version control, instead of THE HORRORS picking up the phone or walking across the hall to talk to fellow employees. Because of it projects are getting bloated, NOBODY is properly double checking with anyone else, conflicts and regressions are becoming MORE common not less so, and on the whole it's become so impersonal there's no life to it any more.
Something I had drilled into me in the '80's was that if you write a function DOCUMENT its calling conventions and return values in a separate text file like you were writing a manual. Nobody seems to do this anymore and the result is a lot of staring at code going "what the **** is this and what does it even do?!?" a few months later.
Said situation is only exacerbated by the sheer number of "Twitter Generation TLDR mouth-breathers" who likely won't even get this far into my response to realize I insult them in it. The people who go "280 characters now on Twitter? What is anyone going to do with that much?!?" -- as a rule we have a lot of people who lack the comprehension, understanding, or literacy to meaningfully contribute to any project.
I'm going to add a FOURTH one... I'm half joking... but ONLY half.
4) Do they swear, a lot... incessantly? I don't trust a programmer that doesn't ****ing swear. You take some simple non profane words like crap or garbage and throw them into the conversation, use a few similes that RESEMBLE a bit of harshness; like something Gordon Ramsey would say.
This HTML has so many blasted pointless classes it's pre-med!
If they get their panties in a wad, make sure the door hits them in the arse on the way out.