How do I find out a blocks ID?
You just need to look for the hook_block() implementation done from the module for which you are invoking module_invoke(). The last argument in the function call you reported is the $delta parameter that is used from Drupal to identify the block within the module.

For example, looking at the code of forum_block(), the part of the code that implement the "list" operation, 0 is the delta parameter for the "Active forum topics" block, and 1 is the delta parameter for "New forum topics" block.

The code you wrote in your question is invoking the implementation of hook_block made from a module whose short name is menu_block (which means the function implementing the hook is menu_block_block()). If you meant to invoke the implementation of hook_block() made from menu.module, then the code should be the following one:

$block = module_invoke('menu', 'block', 'view', 26); 
print $block['content'];


The function implementing hook_block() is menu_block(). The values used as $delta from that function are the values of the "menu_name" field used in the "menu_custom" table.