Websites such as Reverb.com offer price guides that give a general range of a guitar's value. Another option is to seek an online guitar appraisal company like Creamcitymusic.com.
Price researching the instrument on a site that sells them, like eBay, may also provide insight into the market. However, guitar value is not an exact science and there are several factors that affect it such as the make, model, condition and if the guitar is an original. Also, the age of the instrument is important because some collectors value guitars from certain eras or ones that particular artists used on stage.