Walking up to your Lubavitcher neighbor and asking him whether he davens to his dead rabbi is about as useful as asking a used car salesman whether the sticker price is a good deal.  Just what do you think he's going to say?  Even if he would acknowledge to himself that he does daven, he can easily justify hiding the information from you because, after all, he doesn't really "daven", but is, instead, "betten" or, even better, involved in "hiskashrus".
Similarly, don't expect reliable results from questions like "do you believe the rebbe is Moshiach?"  Which normal Lubavitcher would put his job or reputation on the line by admitting that to a misnagid?  If he's worried about the consequences, he, too, will say "no", while thinking to himself, "sure, he might not be moshiach yet, but he will soon" or "I don't believe he's Moshiach, I know he is."
Ditto for "is your rebbe still alive?"
And who can blame them for being evasive?  They know what's at stake and what has to be done to avoid bad PR just like a mechanech in a yeshiva katana knows to skip certain sensitive Rashis when teaching lower grades.  Those kids aren't ready for it yet.  It's called self-censorship and everyone does it.
So, when accurate identification is essential, what's to be done?
First of all, be clear about what you want to find out and why.  If you're interviewing someone to know if he's fit to provide you with shechita or StaM (or provide your wine etc.), his attitude to hiskashrus is probably the best indicator.  If you want to know if, let's say, his summer day camp (or school) is appropriate for your child, you will probably also want to know if he's going to be teaching about his "Moshiach".

We'll address hiskashrus first. 
It's preferable for the conversation to take place with at least two other people present who will understand what's going on and be able to repeat it later.  At the same time, we must consider the Lubavitcher's feelings and try as much as possible to avoid causing him any embarrassment.
Next, ask him how he understands the Rebbe's "atzmus u'mehus" sicha from 5710 (you might like to have a copy with you).  Ask specifically if he feels the Rebbe was correct when he permitted a chassid to "betten" his (dead) rebbe because it's the same as speaking to atzmus HaShem. 
If the Lubavitcher replies that it's complicated, ask him to explain how he understands it, as it's his mindset that we need to understand right now. 
If he dismisses the sicha by saying something like "it's no different then Moshe when the Shechina spoke from his throat", ask if he understands the huge difference between Shechina and Atzmus (Shechina is a finite and temporal presence or manifestation of HaShem, Atzmus is, well, Atzmus.  We can't even discuss Atzmus)?  Be clear that you don't accept that as an explanation of the sicha.
If he tells you that the sicha is only advising us to ask a tzadik to daven to HaShem for us, point out that the rebbe himself, in his footnote to the sicha, acknowledges that this is an idea that he hadn't seen anywhere in any books of chassidus.  But if it's only about asking a tzadik to daven for you, why would you need Chassidic books?  What's wrong with Chumash?  Didn't Aharon Hakohen ask Moshe to daven for Miriam?
In any case, you can tell your Lubavitcher, it's perfectly clear from the whole language of the sicha that the rebbe was equating himself (and his shver) with Atzmus.  This wasn't about davening to HaShem, but about being HaShem.
We would suggest that unless your Lubavitcher is willing to admit that the sicha is forbidden to follow (and the rebbe was badly mistaken), he's probably a card-carrying min.  If he does admit it (using language like "yes, that's always bothered me, too"), then you might ask him why he's still part of a movement that so widely teaches such things...and why, if he admits that it's wrong, he encourages other Jews (including his own children) to live such a lifestyle?

Now what about messianism? 
This one's easier.  Ask your Lubavitcher whether it's impossible for Rabbi Menachem Mendel Schneerson, after his petira, to be Moshiach and that Moshiach will therefore have to be someone else.  If he replies "well, you can't say it's absolutely impossible..." then you can be confident he's a believer.
If he answers that "we don't like to point fingers.  We only want Moshiach to come soon - whoever he is" then he's being evasive.  He hasn't answered your question at all and you have a right to wonder why.
We believe it's unfortunate, but absolutely essential that modern Lubavitchers hoping to provide religious services to frum Jews be subjected to such an examination.  But even if you go to the trouble of questioning him (or her), make sure that you do it properly and don't simply add to the clouds of confusion already swirling around our nation.