Check the cover of the DVD. Widescreen and full screen editions are usually labelled as such across the top of the front cover of the box as "Widescreen Edition" and "Full Screen Edition" respectfully.
Consult an online shopping site. Many online stores that sell DVD's include a cover shot of the movie which will include the widescreen or full screen labels.
Read the DVD's product details online. If a cover shot of the DVD is not available, read the product description beneath. Among other details such as the studio that produced the movie and the film's original release date on DVD, the film's aspect ratio, either widescreen (16:9) or full screen (4:3) will often be listed there. You may need to ask to see these catalogues while some stores place them in the sales area for shoppers to freely browse.
Consult the film distributor's catalogue. Your local DVD rental store will have access to catalogues from all of the major film distributors, either online or through print copies, as that is what they use to order their store's stock. If the DVD is available in widescreen format, it will most likely be listed in one of those catalogues.