I am new to workflows and trying to create one for our project site.  These projects are always set up to revise specific "drawing numbers".  We have had issues where more than one project was created to revise the same "drawing number."  I want to create a workflow so that when a new item (project) is being created, it checks two lists on our SharePoint to make sure it's not already listed. 

If the drawing number already exists, then I want to throw a flag or error.  If it does not exist in the list, they can continue with no issue.

Please forgive my workflow ignorance, any help is appreciated!

Hello,

You can easily start workflow based on condition or data. Either you can create designer workflow OR code based solution to achieve this. Since you have posted in development forum so lets go with code based solution.

You can first create a workflow (any type based on your need) and then set it to manual start. Later create a event receiver to check the lists value and start that workflow via code.

Refer this thread for your ref:

http://sharepoint.stackexchange.com/questions/83125/is-it-possible-to-start-a-workflow-only-when-a-specific-column-is-changed

Let us know in case any doubt