According to the recent patch notes,

We Need to Go Deeper/Mom’s Shovel/Ehwaz now feature a secret method to guarantee a crawlspace once per floor and no longer create them based on random chance

What is the "secret method"?

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1 Answer

You need to search for weird/unusual patterns on the floor and then dig there.

These are some examples:
(in order: Basement, Caves, Sheol, Womb, Cathedral, Chest)

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Womb is probably the easiest place where to find them, due to the red palette. While in Flooded Caves is nearly impossible to distinguish patterns underwater.

See this video for reference.