There are many reasons you might want to know where someone works. You might want to double check that what someone put on their employment application is accurate. You might need to file a lawsuit against someone and need to know where to send the person issuing the subpoena. You might want to catch up with an old friend and want to run into him in public near his work rather than making it seem like you're stalking him. Whatever the reason, there are a few different methods for finding out where someone works, and you might need to try a variety of resources to get the answer.
TL;DR (Too Long; Didn't Read)
If you want to find out where someone works, try using social media sites, Google, people search websites or private investigators.
Do a Social Media Search
These days, many people put everything on their social media accounts. A quick search on Facebook or LinkedIn could help you find not only the person's profile but also where she works. Even social media sites that are more image-focused, like Instagram, can give you a clue to where she works if she put a link to her company's website on her profile or if she posts images from her workplace or company events.
Search on Google
This will be a lot easier if you're looking for someone with a unique name that isn't shared by someone famous. It might not be easy to find a specific "Tom Johnson" in New York or a "Pamela Anderson" aside from the celebrity. On the other hand, finding a "Felix Manchester" in Ontario, California might be a little easier.
To help narrow your results, use a few Google hacks to make your results more specific. For example, when you search for a person, put quotes around his name. You may also try adding his middle name, middle initial or the city and/or state where he lives. It's worth noting that if the person doesn't use his middle initial, adding it might not help your results.
If you're getting too many results for someone with a similar name, you can try removing results related to that person by choosing words that seem to apply to the other person with that name that wouldn't apply to the person for whom you're searching and then adding a dash followed by that word to subtract it from the search. For example, if your search results for "Valerie Flint Portland Oregon" keep giving you results for a dentist with the same name, you can try adding "-dentist -teeth" to the search to see if you can exclude the dentist from the results.
Try a People Search
There are many websites online that allow you to search for people online. You can find a lot of information on free people search engines such as Pipl and PeekYou, which may include a person's current or past employers.
Intelius is a paid people search website, but you can search for a person's name and see in their results a list of many places at which a person has worked before paying to see the person's more detailed profile. TruthFinder is highly rated as one of the most detailed public records search sites, but to see where someone works, you will have to subscribe to their service.
Hire a Private Investigator
If you're really serious about finding out where someone works, and the methods above leave you empty handed, you might need to turn to a private investigator. They are not cheap, but they have access to many databases that the average person can't use and may also take the time to search through nondigitized public records in person, like those available at county courthouses.