It depends. There are many different types of online education, and, depending on where you received your degree, it may be more or less apparent to your employer that it was through a distance education program.
Typically, you will receive one of two types of online degrees: a degree from a university that is based solely online, or an online degree provided by a brick-and-mortar institution. Online and traditional degrees are identical in nature, so they don't show up differently on transcripts, or entail different kinds of diplomas. The only way your employer will know that your degree was online will be if they know about the particular school you went to.
If your degree is from an online university, meaning an institution that typically does not have face-to-face classes or a traditional central campus, your employer will know that you earned your degree online if he or she knows that your particular school was an online school. Some universities, such as the University of Phoenix, are well-known online colleges, and many employers would recognize them straight away and know that your degree was earned through distance education. Other online colleges are less well-known. In this case, your employer may assume you attended a traditional school, or may do some research and find that your school was online. Either way, your degree and school name will look just the same on a resume as a traditional degree because there is no difference between the two.
If your degree is from a traditional university that offers some degrees online, then your employer will probably not know that you received an online degree unless you specifically tell them or they do some in-depth research. Online degrees awarded from traditional universities are completely identical to their traditional counterparts. Online students' diplomas and transcripts are exactly the same as those of the traditional students who attend that institution. Schools do this because they guarantee the quality of their online programs to be up to the same standards as their traditional programs. For this reason, there is no need to differentiate the degrees on transcripts or diplomas. Some traditional universities offer their online degrees through a specifically-named school or program. In this case, it may be possible that your employer recognizes that particular program as a distance learning program, but that would depend on how familiar he or she is with the university you attended.