A common definition of the "true church" among Protestants is that it may be identified by three "marks": the preaching of the gospel, the right administration of the sacraments, and the proper exercise of church discipline. Aside from their historical novelty (at least in comparison to the four creedal "notes" of the Church - that it is one, holy, catholic, and apostolic), there are problems with at least some of these "marks" that really seem to make them unsuitable as a means for identifying "a true church."
With regard to the first, it depends first of all on a definition of what exactly the gospel is. The Catholic Church proclaims the gospel faithfully, but the average Protestant might not agree that she does. This fact indicates that this first "mark" is toting baggage that isn't declared up front, for it depends upon an idea of the gospel's content.
The obvious question that comes to mind is, "what is the gospel that a 'true church' proclaims, by which it may be identified as 'true'?" And of course a second question is, "Who says that is the definition of 'gospel' that we must use?" Hence we see that this "mark" reduces to subjectivity, and consequently doesn't seem to be terribly helpful.
The second "mark" doesn't fare much better, unfortunately. A lot is left unsaid, and the devil is in the details, so to speak. At least some Protestants might reject the Catholic Church outright on this "mark" for no other reason than that the Church recognizes seven sacraments (whereas most Protestants say there are only two). Even setting that aside, what qualifies as "right administration" of whatever the sacraments are? Does that allow for multiple baptisms? Does it allow for grape juice? Does it involve particular understandings of the meaning of each sacrament, and if so what are those meanings? It's totally inadequate to appeal to the Bible about this, since Protestant opinion varies wildly. Here too then it appears that this "mark" doesn't really help.
The third "mark" is fraught with similar issues. Who decides what merits the exercise of discipline? Are any "lapses" at all of discipline sufficient to tag a denomination as a "false church," or is there a threshold? If there is a threshold, who says what it is? Does this apply only to denominations, or does it also (or instead? Why? Who says?) apply to specific congregations within denominations?
Who has standing to declare a particular congregation or denomination as a "false church?" Is it just anyone? If not, who is it?
Given these problems, it seems pretty clear that these "marks" aren't really helpful for more than a "rule of thumb" evaluation; even if judgment is passed on their basis by some ecclesial body of some sort, one wonders why anyone not somehow subject to that body would pay any attention to its opinion.
It seems to me that the creedal notes of the Church are a much better way of taking the measure of any group that claims to be "the church," and have the authority of an ecumenical council behind them (really, of several councils, since at least the first few of them ratified the decisions of previous ones by affirming all that prior councils had taught). This doesn't mean that there are no difficulties involved in applying even this definition, but it's not so fraught with difficulties as the "marks".