Is this a file load or a direct connect import?
It sounds to me that you are asking for a check before import to evaluate if it should even bother with the import at all
So, if using a file based import a parser rule is good for such a thing, a parser rule is a rule that will run on a record by record basis and is already part of the import, so your import would have already started when it runs, and it will evaluate each record on its own merit. This works fine if the evaluation you want to perform is on the first line, but for anything else it will likely import some items and some not using this method.
If you are using a direct connect method, you can evaluate the dataset you are wanting to import before even initiating the import at all, and you would need to do this in the connector business rule. Alternatively, if its connector based you could even make it a requirement for the data to even be returned at all if the criteria is not satisfied (saving the time needed to even transfer any data in the first place) by using a where clause (for SQL) or parameter if using an API.
Joakim