In this case I like to have the guy really lean in and do the push up because many times, especially in a video demo, it can be hard to tell if the demo partner is really even applying force or not, or how much. at least in this case, it's obvious that force is indeed being applied.
Here's a vid of me showing a push test. There's a part in there where we show that the pushing person is really pushing. Basically, I move aside without giving notice. If the pushing person just stands there, you can see on the video that he wasn't pushing. On the flip side, the pushing person can pull away at any time, without warning, and if you move, then that's a good indication that you were fighting the force and not using internal skills.
For the aikido side of things, no resistance in aikido. So, if the pushing person pulls away quickly and you don't move, then you had no resistance. However, if you do move, then it's an indication that you were using muscle to push against the incoming force and you had resistance.
Here's another push test we work on. Since we're all beginner's at this, we start slow and easy and build up to harder pushes. We didn't do the pull away quickly thing because I wanted to show the exercise more than how much effort was being exerted (which wasn't full force, btw.)