The question asked is whether the following question is decidable:

Problem  Given an integer $k$ and Turing machine $M$ promised to be in P, is the runtime of $M$ ${O}(n^k)$ with respect to input length $n$ ?

A narrow answer of "yes", "no", or "open" is acceptable (with references, proof sketch, or a review of present knowledge), but broader answers too are very welcome.

Answer

Emanuele Viola has posted a proof that the question is undecidable (see below).

Background

For me, this question arose naturally in parsing Luca Tevisan's answer to the question Do runtimes for P require EXP resources to upper-bound? … are concrete examples known?

The question relates also to a MathOverflow question: What are the most attractive Turing undecidable problems in mathematics?, in a variation in which the word "mathematics" is changed to "engineering," in recognition that runtime estimation is an ubiquitous engineering problem associated to (for example) control theory and circuit design.

Thus, the broad objective in asking this question is to gain a better appreciation/intuition regarding which practical aspects of runtime estimation in the complexity class P are feasible (that is, require computational resources in P to estimate), versus infeasible (that is, require computational resources in EXP to estimate), versus formally undecidable.

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I have added Viola's theorem to MathOverflow's community wiki "Attractive Turing-undecidable problems". It is that wiki's first contribution associated to the complexity class P; this attests to the novelty, naturality, and broad scope of Viola's theorem (and IMHO its beauty too).