The best thing you can do is look online. If you know the model of the drum, you can look it up and see what Google has to say. It might even show up on the receipt if you still have it hidden somewhere around the house.
However, if that is not the case, here’s what you could do. Find a ruler that is longer than your drum head and measure the diameter of the top of the instrument. Make sure you avoid measuring the metal edges, as those are not taken into consideration. If you own a premium congas drum set for kids, they should never be larger than seven inches in diameter.