There are number of general signs in the SQL Execution Plan that indicate potentially bad performance spots on the query. For example, the most expensive operator that has the highest cost, related to the overall query cost, is a good starting point for the query performance troubleshooting. In addition, the fat arrows, that followed with thin ones, indicates large number of records being processed and flowed from one operator to the other to retrieve small number of records, could be also a sign for a missing index or data multiplication performance issue.
Also, the Scan operators that are used for scanning the overall table or index, indicates most of the time that there is a missing index, the index is badly used, or the query contains no filtering condition. Another sign from the Execution Plan for a performance issue in the query, are the Execution Plan Warnings. These messages are used to warn about different problems with the query to troubleshoot, such as tempdb spills issues, missing indexes or bad cardinality estimates.