How can I tell if something is ruptured inside my lawnmower?
Should be OK
Whenever you change the oil, you have to tip it anyway to get the old oil out.

Your going to have to let it sit for a little bit, might be hard to start up at first and smoke a good bit whenever it does start, but it'll be fine.

Flip it up so you can see under the deck, make sure the blade isn't loose. Just grab it and move up and down.

Right there on the front make sure the spark plug wire is pushed on the spark plug nice and tight, you may want to remove the spark plug and wipe off the oil, dip it in a small cup of gas to clean it off. If it is really bad grab a new one (2-3 dollars maybe?)

The air filter might be bad, it's probably on the left under a black cover.
Should be one screw to remove the cover, pull the filter out. If it is soaked with oil throw it away. You can buy a new one for a few bucks - just go to a store that sales lawn boy and give them the model # of your mower.

Read the book and see how much oil it takes - finish draining the rest of that oil (tip it so it drains out, preferable in a bowl or container and dispose of properly) and fill it back up with new oil.

You should be fine after that, if there is gas left in it of course.