Depending on who made the worksheet, there are various ways to find the answers to high school worksheets. For worksheets found on Education.com, the answer key to the worksheet can be found on the last page of the downloaded worksheet.
Many worksheets have their answer keys provided at the end of the worksheet. For example, some of the worksheets on FunMaths.com feature answers at the end of the document. Similarly, HomeSchoolMath.net offers free worksheets available as either PDF or HTML files. The website states that the answer keys to those worksheets are featured on the second page of the file.