isExternalID() from the Schema.DescribeFieldResult can be used to find External Id fields in an Salesforce Object.

Sample Apex Code:

DescribeSObjectResult sObjectRes = 
	Account.sObjectType.getDescribe(); 
Map < String , Schema.SObjectField > mapFieldList = 
	sObjectRes.fields.getMap();    

for ( 
	Schema.SObjectField field : mapFieldList.values() 
) {    
	
	Schema.DescribeFieldResult fieldResult = 
		field.getDescribe();    
	
	if ( 
		fieldResult.isExternalID()
	) {   
		
		System.debug( 
			'Field Label is ' + 
			fieldResult.getLabel() 
		);  		
		System.debug( 
			'Field Name is ' + 
			fieldResult.getName()
		); 
		
	}  
	
}  

Output: