From my past experience, I would say it's that level of client-to-service-provider that is an ideal match. Location also comes into it, as some people still prefer to have first meetings face to face. 

You can tell if someone or an agency is the right type for you by going through all their recent clients: what type of clients do they normally attract? Are they similar in size to your business? Have these clients got the results that you want to get? (Reviews, Case Studies and Testimonials)

It's also necessary to ask questions of the agency or expert and be specific about something you want to do. See what they say; you'll then know if they are truly helpful or just selling you a package. 

Finding the right marketing firm for you takes time but it is time well worth invested.

Research and explore what agencies present well online and that you feel might work well with you and your business.

Then set up a 'meet and greet' with two or three firms that you identify with. Introduce yourself prior and let them know your intentions (that you are exploring which agency might be best for you).

During the meeting keep it casual and come prepped with questions important in working towards your first six to twelve months of business goals.

From this process, you should have a better understanding of direction and an idea on which agency best suits your needs.

Good luck and I hope this helps!
 

Think of hiring an agency as you would hiring anyone to join your team. Skills are important, absolutely, but how will their people fit with your business objectives, your way of working, your culture?

It's hard to determine this from the outset, so we always offer a 3 month probation or trial period so both sides can get a better feel for the relationship. Don't expect results in this time, it's not always that simple, but you'll at least have a better idea if that partner is right for you.

The first thing to do is get clear on what you want to achieve, get really clear on your goals, your budget, timings, etc
1. Do some online research, check out their website, portfolio and reviews
2. Meet with a few and see which best suits your needs, and is a good match for you and your business. You don't want to be just another number.
3. Go prepared with a list of questions to ask all of them and then work out which one suits. 

You have to kick the tires and look under the hood by talking with them, checking their references and seeing if there is a chemistry fit.   At some point you have to decide whether you trust your gut and  the due diligence and move forward.  If it is not a good fit then you shake hands and part ways and go another direction.  Test and learn.  Course correct as you go.  One firm may not be able to help in all the areas so you may have to divide and conquer based on their strengths.  Good luck.  

Having been in marketing for 28 years when I look to outsource I have some specific criteria.

It all is based around how much the company knows about effective marketing, shown by results. I want to see case studies and hear number based results. 

I like to ask, what figures do you provide or recommend to look at to gauge the success of what I am paying you to do?

Then I drill and ask more questions based on their results. For example on Google Ads it's the Quality Score. If they don't mention its a KPI they use to track success I ask why not.