The simplest way to ask if a company is hiring is simply to call and inquire. However, stopping by with a resume and looking sharp is a more assertive approach that might land you an opportunity even when a company isn't hiring.
Some organizations operate with the mentality, "We are always hiring." This thinking is common in retail, for instance. If you call on the phone, it is easy for a worker to say, "We aren't hiring." But it you stop by in professional dress, act friendly and confident, and ask to see a manager, you may get an on-the-spot interview. Many companies are on constant lookout for talented, quality employees.
If you want to drop by in person, don't do so after a day in the fields or work shop with dirty clothes to pick up an application. Go when you have had time to clean up and put on neat, professional attire, with your resume and cover letter in hand. Rather than ask if the company is hiring, say that you are interested in applying for a job and would like to speak with the manager on duty. If the manager is available, make a quick pitch that lets her know your qualifications and experience, and how you can help the company succeed. At worst, you leave your materials and a good impression, then follow up later. Even if the company doesn't have an immediate opening, an astute hiring manager will remember your initiative and enthusiasm. When a job opens up, you may get a call with a compelling offer.