Some employers test their candidates, but others just believe in what you write. I know of a PHP programmer who listed in a resume a bunch of languages (LISP, Ruby, etc.) that he didn't know for a job that involved PHP programming. The employer didn't check whether he actually knew those languages, and it certainly increased his odds of success compared to his peers who had been more truthful. In the end he got the job.
I think it would be good to have certifications that prove to your employers that you know the languages well. There are well-established certification programs for Java, Oracle, etc. I'm wondering if there are any reputable companies/institutes offering certification in languages like PHP, Perl, Python, C/C++, etc.