An on-site manager only works in and for your HOA. For larger communities that require a lot of attention to management, finances, amenities, maintenance and violations, this is a great option
A portfolio manager takes care of many communities. They work with different boards and manage their finances, maintenance and uphold the rules as laid out in the governing documents
Decide what the board's priorities are. Look for a company that has experience in your priorities' area
If your state has accreditation or licenses for managers, make sure the comapnies' that you're looking at have licensed managers.
Meet the person who could be your manager. Have a meeting with them and see if you would be able to work well together.
Talk to them about how their financial, maintenance and communication processes work
Read reviews on Google, and request to speak to their current or past clients.
When evaluating different companies, price is important. Don't just go with the cheapest, but ensure they fit in your budget.