How to preview blocks
You can manually preview any block within the Shopify checkout editor by copy and pasting the Internal block id into the checkout extension settings. The block must be "Active" or it will not show:
First, copy the "Internal ID" shown on the block detail page within Checkout Blocks:
Next, paste the Internal ID into the checkout extension settings under "Preview Block ID". It's important to note that will override any display rules as it is meant to be a visual and functional preview, not a way to test display rules.
Make sure to remove the block id before saving the checkout editor.
Preview Block with Display Rules
The only way to view a block with the display rules respected is to enable the block on your store. One option is to add an additional rule during testing which requires a specific product.