Check candidate/opponent websites for voting records. Each site will organize the information differently, but often the votes are organized by major issue - like the environment, jobs or healthcare.
VoteSmart.org - Once you've chosen a candidate (presidential, congressional, or gubernatorial), choose the "votes" tab to see how they have voted in the past. Tip: use the "select issue" option to look at the candidate's voting record by topic.
Voting Records in CQ Almanac
In the CQ Almanac, select the appropriate Appendix with voting information. Focus on Party Unity Tables and Voting Participation Tables.