Currently, there are no objective certifications or ratings which can help you find the best Kilimanjaro guides. However, there are ways that you can increase your chances of finding an experienced and well-trained guide.
Firstly, you should book with a responsible climb operator. They tend to have the best guides, because they are better paid, trained properly and given the resources they need to navigate the mountain.
The two things you want to look for are Wilderness First Responder (WFR) training and CPR first aid training, should you fall ill on the mountain.