The insect should be small enough to fit completely into the trap. Any thing bigger you risk the change it might escape or more likely start growing fungus or mold which probably isn't good for the leaf and the plant.
Once the trap seals (2nd phase) there isn't much to see. Before that you can kind of peek in between the cilia (teeth/fingers). A strong magnifying glass is recommended. After it seals you can take a small pen light and light the leaf from behind and you can make out the shadow of the insect (unless it's a fairly light or translucent insect).
Do not try to pry or pull open the trap as you will tear or otherwise damage it.
Remember this is a living organism and one of the true miracles of nature. Treat it as such and not a pet to perform tricks for you and it may live for decades to give you enjoyment for having it.