boolean function to determine whether an commerce order id exists?
There isn't a function that returns only a boolean, but commerce_order_load() returns FALSE if no order is found so you can use it in an if statement quite happily:

if (commerce_order_load($order_id)) {
  // Order exists
}


If you want to be bit more militant about it you could use this:

if (commerce_order_load($order_id) !== FALSE) {
  // Order exists
}


But it shouldn't really make a difference.