The feet, or the spongy, airy ruffle at the bottom of the cookie, are what give it away. Here I can give you some examples of what gives away the hollow.
1. Flat, straight foot
(courtesy of thimbleanna.com & vivianmacaron.com)
This macaron above is hollow. As you can see, there are no air spaces that let the cookie rise up and become fluffy. All of the fluffiness has sank to the bottom while baking, and that is why the foot is straight with no holes. So when you bite into it, the insides are chewy since there is no fluffiness.
2. Tall, lopsided feet
Tall feet means that most of the interior of the cookie has exploded out of the shell, causing the insides to be hollow. Also, there is no air space between the shell and the foot, so that means it will be hollow, Lopsided macarons are definitely a sign of hollowness.
3. Undermixed batter (nipples)
These cookies also have tall feet, and as you can see, there are little nipples sticking up at the top of the cookies. This means that the batter was pretty undermixed, and that plus the tall feet most definitely will make your cookie hollow.
So, what makes a good macaron then?
1. Short, ruffly feet
Short, ruffly feet indicate that you have made a fluffy macaron. The insides have not spewed outside the shell nor have sank to the bottom.
2. Air space between foot and shell
Air space between the foot and the shell also indicate that you've made a good macaron. You can look inside this air space and see that the meringue has turned fluffy.
I noticed that most macarons that are hollow are usually undercooked. This can easily be fixed by cooking it a bit more, and making sure you deflate the air in the batter correctly. Here is a video showing you how to do it.
This makes a big difference in whether your macarons turn out hollow or not. Air in the batter contributes to hollows. Make sure you get every single area of the batter while scratching the bottom.
To get rid of hollows aside from the batter making, you need to make sure your oven is the right temperature. Raising up the temperature a little higher than 300 (my recipe) if your oven thermometer says it is not actually 300 will help. I bake on the middle-high rack, since my heat is mostly from the top. I use a convection oven as well. You also need to bake them a little longer, like 13 minutes. This will prevent hollows and ensure a stable inside. However, the trade off is that you will have crispier macarons than normal. If you have a high-moisture filling, it will disappear with a day or two of maturation. If you have a low-moisture filling like buttercream, you can additionally brush the bottoms of the macarons with milk or whatever will go with your flavor, like jam or coffee. This will help maturation proceed faster, and your once-cripsy macarons will turn into fluffy, soft, chewy confections.
THIS IS FOOLPROOF. If you follow all these steps and analyze your macarons' feet, you will defeat the hollow problems!:)
Hope this helped.<3