For a series of throws to work as a juggling pattern, on each beat you have to catch one ball, so that you have a ball to throw on the next beat. This fact can be used to check a prospective siteswap pattern.

Every time a 3 is thrown in a juggling pattern, it is caught two beats later and thrown three beats later by the other hand. Similarly a 4 is caught three beats later and thrown four beats later by the same hand. Whatever the height of the throw, the same ball will be thrown again that many beats later. So to check a pattern , if it contains a 3, write 3 under the throw three beats later, if it contains a 4, write 4 under the throw four beats later, and so on. If a pattern contains a 0, write 0 underneath itself! 

When you’ve done this for every throw, the pattern will work if there is one number under each number in the pattern. If there are two numbers under one of the numbers, you would have to catch two balls at once. Alternatively if there isn't a number under one of the numbers, you wouldn't catch a ball or be able to make the next throw.

Here are a couple of examples:
Let’s check whether 51234 is a valid pattern. First write 5 under the throw five beats after the 5:
     51234
     5
Next write 1 under the throw one beat after the 1:   
     51234
     5 1
Now write 2 under the throw two beats after the 2:
     51234
     5 1 2
Now write 3 under the throw three beats after the 3:
     51234
     531 2
Finally write 4 under the throw four beats after the 4:
     51234
     53142
In this case there is one number under each number of the pattern, so 51234 is a valid siteswap pattern.

With valid siteswap patterns, if you read the checking numbers in reverse, that is also a valid siteswap. Often this results in the same pattern, but in this case the pattern is 52413.

Now lets apply the same procedure to see whether 55235 is a valid pattern. First write 5 under the throw five beats after the first 5:
     55235
     5
Next write 5 under the throw five beats after the second 5:
     55235
     55
Now write 2 under the throw two beats after the 2:
     55235
     55  2
Now write 3 under the throw three beats after the 3:
     55235
     55  2
      3
Finally write 5 under the throw five beats after the final 5:
     55235
     55  2
      3  5
In this case there are two numbers under the second and last 5, and there are no numbers under the 2 or the 3. So 55235 is not a valid pattern.