I want to start my workflow only if the value of a particular form attribute is anything other than -1 .
You can't use the EPM-check-object-properties handler to check on 'IS NOT'-values. It is possible with a condition task and a query: property IS NOT -1. Unfortunately, you can't check with condition task before starting the workflow process. So, you need to route to a do task, to invite the user to correct the property or any of the target objects.
Another solution is to check the property before start with a TCPB Workflow Handler by using regex. This is possible with the TCPB (Teamcenter Power Box) AddOn. Available with costs in GTAC only in German speaking regions.