Check for a specific object:

Schema.DescribeSObjectResult objDescribe = Schema.sObjectType.Account;
System.debug(objDescribe.isCustom());

Check for all objects:

List<String> customObjectList = new List<string>();
List<String> standardObjectList = new List<string>();

for(Schema.SobjectType obj : Schema.getGlobalDescribe().values()) {
    Schema.DescribeSObjectResult objDescribe = obj.getDescribe();
    if(objDescribe.isCustom()) {
        String objName = objDescribe.getName();
        customObjectList.add(objName);
    }
    else {
        String objName = objDescribe.getName();
        standardObjectList.add(objName);
    }
}

System.debug('Custom Object List - ' + customObjectList);
System.debug('Standard Object List - ' + standardObjectList);