Finding owners background info

[Alex Verdugo]

I am trying to locate the owners of a few properties I've sent yellow letters to.  I had these letters returned with no forwarding address available.  What is the next step to find these owners?  I'm aware of the many different skip tracing companies that can find the information I'm looking for but I'm one of those people that likes to know how things work.  I'm sure eventually I will outsource this task, when my investing career in much further along.  But for now I would like to learn how to do my own tracing.  I'm sure there are times when I won't need or want a full report from these companies and only need a few pieces of information.  I would like to learn how to fund all the credit reports, criminal records, rental history, etc.  As a for instance, I have a friend who I know has a criminal past. Ive know him for years and he is a good person, mostly.  I wouldn't necessarily need to know his rental history because I know where he's been living the past 4 years.  But some of the other info might be helpful.  

Where can I get a copy of his credit report?  For free?

Is there a free way to find out about his criminal past without manually going to every county in the US and searching his name?

And on a similar note, what all is contained in a skip trace report?

Thank you for helping.