
Then pick up any calculus textbook and chug through it (Thomas is good from what I've heard). Even if the questions don't have answers in the back of the book you can check your computational steps for free using: https://www.symbolab.com.
I also recommend lots of practice, so khan academy is good for drilling + any problem set book with lots of calculus problems, such as "Schaum's 3,000 Solved Problems in Calculus" or "Essential Calculus Skills Practice Workbook". They don't have to be huge calculus text, doing problems is more important than reading through 1,000s pages of colorful examples. You can find shorter calculus books that focus primarily in drilling calculus techniques. Focus on those to nail the techniques.
I agree with you 100%. But let me add this: in most cases, if you're studying with a book that doesn't have an answer key, you can supplement that text with exercises taken from somewhere else. For example, lots of course websites around the 'net post previous years exams / homework with answers. There are also books like Schaum's 3,000 Solved Problems in Calculus[1], The Humongous Book of Calculus Problems[2], 3,000 Solved Problems in Linear Algebra[3], etc.
Also, with books that are used as textbooks, and that provide an answer key but only to instructors... if you aren't averse to violating copyright and using certain pirate websites, those "instructor only" answer keys can often be found.
[1]: https://www.amazon.com/Schaums-Solved-Problems-Calculus-Outl...
[2]: https://www.amazon.com/Humongous-Book-Calculus-Problems-Book...
[3]: https://www.amazon.com/000-Solved-Problems-Linear-Algebra/dp...