rodionovit

import json

json_str = '[{"Id":1267,"slot":0,"codename":"beans","name":"apple1"},{"Id":1268,"slot":1,"codename":"pineapple","name":"potato"},{"Id":1269,"slot":2,"codename":"chil","name":"ball"},{"Id":1270,"slot":3,"codename":"apple","name":"banana"}]'


json_obj = json.loads(json_str)
# если отдельный элемент проверить
if json_obj[0].get("slot") != None:
    print("ok")
else:
    print("not ok")
    

for item in json_obj:
    if item.get("slot") != None:    
        print("ok")
    else:
        print("not ok")