You have some great answers in these responses. I would add that the underlying question is "how do you compare?",
Comparison starts with your unit. It sounds to me as though you are a seller and a buyer, is that right? One of the reasons that sellers contact Realtors is to find out what they can do to maximize the selling price of their current home. Would that be of value to you?
The advantage to asking a Realtor for advice is that they know what sells. They know simple steps to take to make your property look it's best.
The other major task a Realtor has it to expose your property to the market. What is the point in tweaking a property with new paint, a little staging, decluttering, etc. if the photos taken are of poor quality, and only one or two are posted on the web?
Lastly, once you have an expert opinion of YOUR condo, how does it COMPARE to the others. Should you go just by the tax records? Should you visit 20 condos? A Realtor is IN the marketplace, showing property to buyers, writing offers, listing property, negotiating, so they KNOW what has value. Remember, in real estate the three main selling points are location, location, and location. And in condos it also means "what about your building and your HOA?". Adequate reserves, deferred maintenance, proper insurance, smooth runningt HOA? Realtors who are working IN the market know that in order to make an apples to apples comparison, you need to know all the pertinent facts.
Hope this is helpful to you.
I agree with Deborah and the other posters - it helps if you personally check out other similar sized properties in your area through open houses, etc. That way, you'll see what the competition looks like, and you'll get an idea of where your property fits in. But do remember that the sellers' asking price might not be the same as what a buyer is willing to pay. So in any case, speaking with several well recommended Realtors wouldn't hurt - especially since it won't cost you anything more than your time to hear their opinions.
As a property owner, you can collect the data on sales, and tax records and draw your own comparisons. Keep the points in mind from the prior paragraph. Try to look at the strengths and weaknesses of your proeprty from the view of third party. What amenities does your property have? What is the condiditon compared to the comps?
A CMA from a Realtor will provide you with a current snapshot of the marketplace. It will show Active listings (your competition) on the market, those that have recently gone under contract and recent Closed sales. The CMA can be tailored so that it shows how your condo compares to similar condos within a certain price range and geographic area.
Good luck, Ken.
To summarize- the local MLS will hold the answer.