"Spanish Is Fun - Lively Lessons for Beginners" is a Spanish text book by Haywood Wald. There are both English and Spanish versions, so it can be used for English speakers who are learning Spanish or for Spanish speakers who are studying their own language more in depth. The book is both a textbook and a workbook, and the answers for the workbook questions are found within the text of the teacher's edition. However, if you don't have the teacher's edition, you can use other methods to find the answers.
The "Spanish Is Fun Teacher's Edition" is the best way to find answers for "Spanish is Fun." The teacher's edition is the exact same book except the answers to all of the questions are listed in the same place as the questions themselves. If you follow along with your students in this book, you can have the answers right at your fingertips.
Make Your Own Answer Key
If you don't have a teacher's edition you can make your own answer sheet. If you know Spanish, even at a basic level, you should be able to figure out the answers to the questions as they are at a beginner's level. Make a word processor document or even handwrite the answers on a sheet of paper before you teach each lesson.
Use a Native Speaker
If you aren't confident of your own Spanish knowledge, or if you are using the book on your own and are not a teacher, ask someone who speaks Spanish to look at the lessons and see if you have the answers correct. A Spanish teacher, exchange student or member of the community who is fluent in Spanish can quickly look at the answers you've written and correct them for you.
It might seem silly, but you might not need the answers at all. The book is a text book and a workbook, so you can get plenty of good information out of the book, even if you don't have the answer key. Just try your best on the exercises and continue to go through them for practice. Even if you aren't doing everything perfectly, you can still learn quite a bit of Spanish.