isCustom() method from Schema.DescribeFieldResult can be used to find whether the field custom or standard in Salesforce using Apex.

Sample Apex Code:

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

for ( 
    Schema.SObjectField field : 
    mapFieldList.values() 
) {    
  
    Schema.DescribeFieldResult fieldResult = 
        field.getDescribe(); 

    if ( fieldResult.isCustom() ) {

        System.debug(
            'Field Label is ' + 
            fieldResult.getLabel()
        );
        System.debug(
            'Field Name is ' + 
            fieldResult.getName()
        );

    }
      
}

Output: