If unsure, look for copyright notices on the site, or e-mail the web master. But there can be complications. The most frequent problem is bad pagination. Things that look good on screen don't always fit on A4 sheets neatly.
Some sites provide special pages that are more suitable for print.
Some sites also provide material for print in what are known as 'PDF' files - files that must be read by Adobe Acrobat Reader. This may seem awkward, but it is in fact the most widespread way of publishing material for print over the web. To read this material, you must have Adobe Acrobat Reader installed. PDF files are often used for printable forms because the resulting page will print much closer to the designer's intentions.