Question: If I am messing around with XCode and trying to learn Objective-C, does it pose any threat to my system if I routinely forget to release objects? What is the real consequence of not releasing objects? If I restart my computer will these memory leaks persist?

I am mostly messing with NSStrings and NSTimers and things like that, nothing serious. I'm just concerned about it messing up my computer.

Is there any way to check if there are unreleased objects? What is one to do if they want to check a bunch of code to see if there are any of these memory leaks?

Answer: 
The only "threat" is that your program will eat up a large chunk of your available memory, which will cause your computer to go slow and your hard drive to thrash until the system reclaims the memory when the program ends. It makes things pretty unpleasant when a program uses more memory than it has to. Not dangerous — just really annoying.
Don't worry, any memory you leak will be reclaimed by the OS when your program ends (or crashes). 

The leaks and object allocation tools in Instruments will help you find your memory mgmt bugs. Also, the static analyzer (Build->Analyse) is very helpful.