How to identify look up relations to an object in Salesforce

You could get this with apex describe methods:

for ( Schema.ChildRelationship scr : Account.sObjectType.getDescribe().getchildRelationships() ){
    system.debug('');
    system.debug('**** Child SObject         **** ' + scr.getChildSObject());
    system.debug('     Field                 **** ' + scr.getField());
    system.debug('     Relationship Name     **** ' + scr.getRelationshipName());
    system.debug('     Cascade Delete        **** ' + scr.isCascadeDelete());
    system.debug('     Deprecated and Hidden **** ' + scr.isDeprecatedAndHidden());
    system.debug('     Restricted Delete     **** ' + scr.isRestrictedDelete());
}

Or if you want a visualization of the object relations, the schema builder is a nice tool.

There are couple of tools thats very helpful

1.Workbench

You can see the Childrelationships to see all the all the objects for which the object is looked upto

2.Appexchange app named Schema Surfer

This app has logical arrangement of your metadata to help you figure all child objects

You can also use Etherios EasyDescribe - Free MetaData Viewer/Extractor from AppExchange which is shown below. You can also export the result to excel format.

And also it has many other features.