To access the standard Object Limits page, navigate to Setup | Customize. Click on the name of the desired standard object, and then click on Limits, as shown in the following screenshot (for the Account object):

Here, you can see usage details for these: Custom Fields, Rollup Summary Fields, Custom Relationship Fields, Active Workflow Rules, Total Workflow Rules, Approval Processes, Active Lookup Filters, Active Validation Rules, VLOOKUP Functions, Sharing Rules (Both Owner- and Criteria-based), and Sharing Rules (Criteria-based Only).

To view information about the usage of various fields and rules that have been created on a custom object, you can access the Object Limits window displayed on a custom-object-definitions-related list at the bottom of a custom object definition page.