How to know if I am doing something an idol?
Good question (though by "something an idol" I am taking you to mean "idolatry" or the like).
On the one hand there are clear boundaries when for example one starts praying to a god other than the One in the Nicene Creed (e.g., Krishna). I would call that idolatry, one that touches on basic beliefs and religious commitments.
On the other hand, there are many things that probably are not going to be a problem when it comes to idolatry (e.g., is one really going to make an idol of Argon gas or the letter "m" and so on?). It's not the clear but the grey areas that are tricky.
As a Protestant--well, my response here may be more broad in various respects--I think one often ought to hesitate to make clear judgments against others. I have my doubts about the pastor's judgement-against-car-buyers illustration in the OP, not that the pastor is necessarily wrong in some case he may have had in mind, though that may be for other reasons than stated.
Anyway, being cautious about others (and sometimes about oneself) may be a sort of rule of thumb.
Some rules of thumb (at least as I see them) or tests have already been mentioned.
To whom or to what does one turn in an emergency? Not that calling "911" when a child passes out cold on the kitchen floor is sure sign of idolatry either--I'm talking rules of thumb only. Do we even think of God in the early moments of a car accident?
Obsession. This may be more of a long term indicator. Lines between obsession and "joyful, natural inclination" or other such thing may get blurred, and there may be room for focus such as in a career or crisis when ordinary routine must yield temporarily to concentrated, single-focus effort.
Relative importance to oneself. Of course there are tasks that need to get done, many important or needful. The border between duty or need on the one hand and idolatry on the other may not be clear, nor quite what importance is. But yes, when "what God is not" becomes more important to oneself than "what God is," that is idolatry.
Deprivation. I mean, how does one react when deprived of something. This may be a test, but I doubt it is foolproof. Is irritation at deprivation a sure sign of idolatry? Does such irritation lean toward idolatry, but not necessarily get there?
Fear. I mean, how does one react in fear-inducing situations. Not that one should be apathetic when in imminent danger or not take precautions to avoid hazards. But there are places that show us whom we fear more, God or something else. "The fear of man is a snare ..." Prov. 29:25.
What about visual images? Here is not the place to go into the history of iconoclasm. There are pros and cons here as above. All I can suggest is caution and rules of thumb for a lot of the grey area issues.
One might wish I had clearer boundaries to a lot of this; in this life we are left in many areas and cases to judge for ourselves whether we love God with all our heart, mind, strength, etc., while also He gives us all things to enjoy and thank Him for. Yet for all my quibbles and qualifications, the problem of idolatry in the Christian life is important enough that John saw fit to issue a warning as the last words to his first epistle: "Little children, keep yourselves from idols." Thank God for Jesus Christ our Advocate when we sin!
