Do you have a large block of data that you want to check the structure of?

PARSE is a good way to do that, since it returns TRUE if it can parse the entire block properly. PARSE happens also to be very fast.

Our data should be a block of blocks:
data-block: [
[1 2 3]
[4 5 6]
[7 8 9]
]

So you can simply check that each entry is a block:
>> parse data-block [any block!]
== true

Let's sabotage our data block:
data-block: [
[1 2 3]
none ; oops!
[4 5 6]
[7 8 9]
]

>> parse data-block [any block!]
== false

Simple!