.trying to update a bunch of old vendor bills. noticed sometimes they have expenses and sometimes they have items. what's the best to get line item type?

A vendor bill may contain both so better to check if both are existing.

Thanks. But is it possible to get the line item type like we can for records?

I don't think there is. You normally get this by calling nlapiGetLineItemCount(type) and explicitly stating what 'type' it is.