It appears from some of the narrations that there are three inevitable signs; they are: the appearance of the sun from the west, and the cry from the sky that will declare the reappearance of the Imam, and the swallowing of the earth in the desert. Other signs are possible.
There is no doubt that Almighty Allah will establish undoubted clear evidence when he reappears, so it is better to delay paying attention to these issues to their designated times and not rushing to conclusions in this Godly matter, since He will clarify it at its appropriate time.
It is permissible to use the miswak or the like when fasting, but one should avoid swallowing the liquids from outside the mouth. For example, when one uses the miswak and it becomes wet from saliva, if it is taken out of the mouth and placed back in the mouth again, the wetness on the miswak will be deemed to be from outside the mouth, and swallowing such liquid will invalidate the fast.