While I am *not* a law enforcement officer (of course, if I were, I'd be allowed to lie and say I wasn't, so you never know), I incidentally know a lot about this.
I can tell you that asking someone if they are LE is a dangerous myth. In many cases, the FAILURE of LE to lie in that circumstance is, in and of itself, a crime. They MUST lie to you or be charged with a crime themselves.
Furthermore, if you are under any form of surveillance, the associated phone/internet or housing providers are barred from informing you of that surveillance. It is a crime to inform you.
Do not believe, for one moment, that there is any single question you can ask of anyone to determine whether or not they are LE. (Unless you use enhanced interrogation techniques as part of your screening process.)
In addition, as certain things may constitute a form of "organized crime," non-Law Enforcement informers are often used.
If, for example, I were trying to end hobbying from the demand side; I would find a popular and highly reviewed lady and lean on her -- hard. I'd get an indictment on a host of things (ridiculously easy) and have her looking at 1,000 years behind bars for starters; then have her agree to have all of her work videotaped for a month as she provided all phone and email records. In exchange for her cooperation, the charges would be dropped and then the arrests of prominent businessmen and politicians would begin ...
You are never, ever, safe if you are engaged in anything that might even be misconstrued to be illegal.
I know a man who has politically incorrect views. You can't be put in jail for your thoughts directly (yet), but LE can be creative. Early in life, he was charged with a misdemeanor for theft and served a year in jail.
Later on -- 10 years later -- he was put in jail for five years for being a "felon in possession of a firearm," even though he had never been convicted of a felony. The rationale was that, as his misdemeanor COULD have resulted in a sentence of longer than a year, it should count as a felony for purposes of interpreting federal law. The feds spent over a million dollars following the guy around, and in the end, that's the best case they could make.
He simply agreed to the charges rather than fight, because the prosecutors told him that if he dared to take it to trial, they would charge his wife with "conspiracy" and his kids would end up wards of the state while husband and wife rotted in jail waiting for a trial. (Bob Barr, recent libertarian candidate for President, was his defense attorney. The man is now out of jail and working as a personal trainer.)
I also know a guy who is an organic farmer who was charged with "animal cruelty" for killing rabbits that were raised for food and making an instructional video of the process. This was also because of his opinions, but he managed to beat the charges after thousands of dollars in legal bills. The indictment was secured by calling his livestock a "pet bunny." Just a little twist, but legally significant.
This is the bottom line: if LE wants you bad enough, they will get you. They can lie, pressure, cut deals, twist laws and all kinds of stuff. Maybe, MAYBE, you can secure justice if you have enough thousands to spend on lawyers.
The one thing I have learned, in general, is that the nail that sticks up gets hammered down. If you want to avoid this sort of thing, don't be blatant. Don't be in their face. Don't defy them openly.
Millions of Americans (including many politicians) cheat on their taxes with no consequences. Irwin Schiff published a book and website advertising the fact he didn't pay taxes and even ran seminars. He was a nail that was sticking up, was in their faces and defied them openly. He also went to the slammer.
Don't be Irwin Schiff.