(somewhat off topic, but a majority of method 1/J/K PIDs occur on many seeds, thus dates, so date checking is somewhat inconclusive)
how to check, pretty roughly
As I told Guested in IRC, there are ways to find a seed in which the egg was generated, but it takes a hell of a long time to check.
86% of all possible initial seeds are humanly impossible (delay 500-10000, shiny egg RNGers go to at max 3-5k, where the egg is the first egg). If it's not a shiny egg, I just change the SID to make it shiny (removes 8191:1 result of PIDs) and find all possible shiny eggs in that delay range. You can further refine it by specifying the gender ratio and ability (removes half of the remaining, and even more with the gender ratio). So you get just a handful of PIDs. It takes a hell of a long time to get them all. Do it for both Normal and International egg PIDs.
For it to be useful flawless, it'd have to be RNG'd, so there would be no everstone used. There is a PID algorithm for the egg based on the IRNG method. International Egg PIDs's method is slightly different.
Once you have that spreadlist, you can see if that PID occurs. If it was an RNG'd egg (which is the case here), it must be one of those 2.
It's flawed in the fact that the Everstone influenced PID method isn't public(ly known), the person can forgo checking their initial seed and just manually test eggs until they are the desired result.
There is a PID method, it's hard as hell to check currently...
on topic:
Just don't use a method 1 PID. If you really don't think someone would be so hellbent on checking it, you can go with a random PID. Usually snagging one from RNG reporter is fine.