Search online and check the Better Business Bureau for company ratings. You can also look at customer reviews, but keep in mind that some might be created by the company or paid advertisers.
If you've seen a presentation that impressed you, contact the company and ask who they used. Word of mouth is the best recommendation!
Always verify a business’s credentials.
Another option is to look for schools that teach advanced PowerPoint classes - they might have connections to good companies.
Good luck!