Do Demons Produce Strong Physical Evidence That They Are Leaving?

Not always or even most of the time.  You must NEVER rely on a physical manifestation to prove that a demon has left.  In both the Bible and my personal experience, there are plenty of cases where there was NO manifestation – they just hit the road in the name of Jesus because they had to.  I’ve had cases where I discerned spiritually that a demon had left or saw it leave through Spirit-granted vision when there was nothing in the natural to indicate that it had left, other than that the torment of the person stopped and the person suddenly felt “lighter” instead of feeling beaten down.  The Bible makes it clear that it is POSSBILE that someone could fall over and stiffen up such that people think the person is dead (Mark 9:26).  However, in the MAJORITY of Bible cases as well as ones I’ve seen myself, there is NO “manifestation” at all.

In fact, I can also answer the question by saying that sometimes the evidence of their departure is the CESSATION of strong manifestations!  For example, people who are slivering on the floor acting like snakes (you’ll see this in some countries) suddenly STOP manifesting a demon when it leaves.  When Paul cast out the spirit of divination out of a certain slave girl (Acts 16:16-18), the girl’s fortune-telling business came to an abrupt end, and that was obvious to her masters:

Acts 16:19:
And when her masters saw that the hope of their gains was gone, they caught Paul and Silas, and drew them into the marketplace unto the rulers,

The erroneous teaching that you HAVE to see some manifestation has led to counterproductive and dangerous services where people are taught that they have to vomit up demons into what appear to be Kentucky Fried Chicken buckets with the labels taken off.  I had the misfortune of being in such a service when no one warned me in advance that the church in question practiced that.  If you’ve never been to such a disgusting, revolting service, consider yourself blessed.  The more a church does that kind of thing, the more similar “manifestations” it will see, convincing the people even more that this is a God-given revelation of how to deal with demons.  What REALLY happens in those services is that well-meaning Christians actually yield their flesh to demons and ALLOW them to make them sick.  Satan is always OK with the idea of making people sick, and I imagine that he appreciates the unwitting cooperation.  If you didn’t even HAVE a demon when you came in, you could have a demon mess with your flesh in such a service through your ignorance.  I’ve stuck to dealing with demons biblically and have never at this writing had anyone throw up when I cast a demon out.  But then again, no one was EXPECTING to throw up as they did at the weird service I mentioned.