Asking about the detection status of specific quanta that are predicted in theories of beyond-the-standard-model physics should be fine. Looking for new particles is, of course, a major activity in experimental particle physics. Moreover, there are also secondary observables that are also indicative of the presence of new physics. (For example, the mass of the top quark had already been "measured" to within about 15%, even before it was detected directly in 1995, because of the top's outside influence as a virtual contributor to certain lower-energy particle processes.) When one has a particular quantum field theory model in hand, it is often straightforward to determine what new effects we would expect to see in that model, as a function of its parameters.
Such predictions can also be turned around and combined with experimental data to place constraints on the models (assuming that the new particles they predict have not actually been seen). For example, while supersymmetric extensions of the standard model have a number of very attractive theoretical features, we have never seen the superpartner of a standard model particle. Over the last several decades, increasingly energetic and precise experiments have led to region of the SUSY parameter space that is phenomenalistically viable continuing to shrink—to the point where many feel that relatively-low-scale SUSY is no longer even a theoretically attractive option. There is an extensive literature on these kinds of constraints for SUSY theories and other kinds of extensions of the standard model, and they surely fall within the heading of "mainstream physics."
