Whats the best way to check if a custom attribute exists in Magento2

You need use \Magento\Eav\Api\AttributeRepositoryInterface::get method for this.

For example:

try {

   $attribute = $this->attributeRepository->get($entityType, $attributeCode);

} catch (\Magento\Framework\Exception\NoSuchEntityException $e) {

    //  attribute does not exist

}