How can you estimate horsepower?

I went up into the local hangout spot and got surrounded by these kids i've never seen there before. It turns out that one of them just got their license and wanted to hang out. I used to be a n00b too up there, so I said hey man welcome you know whatever. People surrounded my car when I first came up there but now Im a usual. I pop the hood all the time though for people. Anyway.... I asked what the kid good drive drove. He told me an Integra with 195 horsepower. I didn't wanna say anything to drive the kid off, or to make him think I was a complete asshole, but honestly how can people go around telling others how much horsepower their car has without dynoing it? How can I estimate?

For instance.....

I did an engine swap in my car the new engine specs are like 200 hp at 4800. Then I put in an intake that CLAIMS it gives 5-7 hp. Then, I got new wires that CLAIM they improve performance. Then I got my supercharger that runs 8psi, BUT it says between 240-500 hp gain. Then I got my ported manifold that also CLAIMS to improve performance. Now, how could i take all these claims and statements and give someone an estimate? Like someone estimate how much HP my car has cause at this point I really have no idea.