When is it too early to ground an aircraft, (i.e, how do you know that you're jumping to conclusions or not)?

Some countries have grounded the 737 Max 8, and UK banned it from it’s airspace. The countries who haven’t, are saying that’s jumping to conclusions or similar.
https://tinyurl.com/y5sdf569 (737 Max crash/Google)
https://www.cnn.com/2019/03/12/africa/airlines-suspend-boeing-flights-intl/index.html