There is an organisation callled the CFPMA (Central Florida Property Managers' Association) which is a trade association. They have a code of conduct and their members (management companies) are required to adhere to it. Whilst it is not an absolute gurantee of quality it is helpful and using one of their members is helpful as the CFPMA will do all they can to hold their members to account.
You can also look at the Better Business Bureau (Orlando branch) and see whether the managament company is a member or whether they have had reports about them on that site.
If you are booking direct with an owner they should, if they are proactive, be able to give you some re-assurance about their management company and show you that they are licensed and hopefully give you some very recent testimonials about their management company. A good owner will be able to let you have details of a few recent guests who you can contact to satisfy yourself that the management company is good. A good owner will also ask all gussts to compelete a feedback questionnaire about the house and the management company. Thus, good owners should be able to give you information to help you feel more confident.
This site: http://www.sunbiz.org/ will also give you information about any company that is registered in Florida and all have to be to be legal!