Okay thanks Baikal for all these info. Who should I contact or where should I go to verify if I am a US citizen or not or to plea my case? The last thing I want to deal with is an immigration lawyer cause they'll probably charge me more then actual cost of getting a passport if I am already derived citizenship. Even if I happen not to be a citizen I am gonna have to pay 600 for an easy *** test I know I am gonna ace + lawyers fees? Screw that! You guys can call me cheap but I rather do either one on my own.
There is no place that you can "go to verify if I am a US citizen". All you can do is submit an N-600 application or a passport application and see what happens. Note that even if you do qualify for derived citizenship, your application may easily be denied if you don't provide adequate supporting documentation.
That's exactly why I suggest consulting with a lawyer first.
In your case you have to first decide whether to try to claim that (1) your parents were married in Cambodia or (2) that they were not legally married but that you were legitimated under the Washington state family law.
Because you do not have your parents' marriage certificate, you are going to have to attach a letter to your application explaining exactly why you believe that you qualify for derived citizenship and explaining what kind of supporting documentation you are providing.
For each of these options different types of supporting documents are needed. For option (1) you'd have to provide some kind of a secondary evidence of your parents' marriage. I don't know what kind of evidence is considered acceptable/pursuasive for such cases, and also what if any documents of this kind your parents actually have. A lawyer should be able to clarify this for you.
For option (2) you'll have to explain that you are claiming legitimation before the age of 16 under the former Washington State law RCW 26.26.040. You'd presumably need to find the actual text of that law as it was then (the AAO decision I linked above does not cite that law directly but rather cites a relevant court decision -- I am not sure if that'll be enough). You'll also need to provide some actual evidence of having satisfied the relevant provision of RCW 26.26.040. Presumably something like school records, maybe tax returns where your father listed you as a dependent, maybe some documents related to your GC application, etc. Again I don't know what exactly you have and what would be consdered acceptable/persuasive by the USCIS and the State Department in terms of evidence of this type.
You are certainly welcome to try to do all this research completely on your own, prepare an application (an N-600 or a passport application) yourself, enclose a letter explaining your situation, with supporting documents, then submit the application and see what happens. But I personally would have consulted a lawyer if I were in your shoes (and even then, I would have tried to collect as much documentation as possible first).