[SOLVED] Check if a record has siblings on creation

[Guido1982]

OK, I'm pretty sure we can do this, just not sure how exactly:

I want to create a workflow that runs when a record of module A is created. Records from module A contain a UI10 type field to module B. I want to check if this record (the new, module A type record) is the only one related to module B through that UI10 field. If that condition is met, a checkbox should be true, otherwise it should be false.

Or would it be better to do this through a businessmap and use some aggregation expression?