There's a number of subfields that have a stackexchange site of their own by now, such as linguistics, statistics, computer science, etc. It also appears that questions with certain tags are very likely to be migrated to some field-specific site and/or get a comment proposing to ask this question again on the said site. For example: [machine-learning] or [classification] or [statistics] suggest that the question is suitable for http://stats.stackexchange.com, [linguistics] or [nlp] often point to http://linguistics.stackexchange.com, [algorithm] or [complexity] may indicate relevance to http://cs.stackexchange.com or https://cstheory.stackexchange.com/, etc.
The problem, however, is that many users are not aware of these specific stackexchange sites or have trouble identifying the one they are looking for. Do you think it would make sense to inform the author of the existence/relevance of these resources when they have done the tagging or, possibly, based on the body of the question same as suggested tags?