In several jurisdictions, the actus reus of a criminal attempt is taking an act beyond mere preparation towards the completion of a criminal offence. What is the test for determining when an act is beyond mere preparation?
Maybe it is unsatisfying, but there is no bright line test.
The statutory language
The Criminal Code says:
The question whether an act or omission by a person who has an intent to commit an offence is or is not mere preparation to commit the offence, and too remote to constitute an attempt to commit the offence, is a question of law.
Some judicial expressions
"[A] precise and satisfactory definition of the actus reus is perhaps impossible." And "[n]o abstract test can be given for determining whether an act is insufficiently proximate to be an attempt."
See R. v. Cline (1956), 115 C.C.C. 18 (Ont. C.A.):
The consummation of a crime usually comprises a series of acts which have their genesis in an idea to do a criminal act; the idea develops to a decision to do that act; a plan may be made for putting that decision into effect; the next step may be preparation only for carrying out the intention and plan; but when that preparation is in fact fully completed, the next step in the series of acts doen by the accused for the purpose and with the intention of committing the crime as planned cannot, in my opinion, be regarded as remote in its connection with that crime. The connection is in fact proximate.
Here is a summary from R. v. Root, 2008 ONCA 869, citations removed:
 The authorities have yet to develop a satisfactory general criterion to assist trial judges in making the crucial distinction between mere preparation, on the one hand, and an attempt on the other. We leave the determination of where on the continuum the conduct lies to the common sense judgment of trial judges.
 The distinction between preparation and attempt is a qualitative one involving the relationship between the nature and quality of the act said to constitute the attempt and the nature of the substantive offence attempted in its complete form.
 To determine on which side of the preparation/attempt divide an accused’s conduct falls, a trial judge should consider the relative proximity of that conduct to the conduct required to amount to the completed substantive offence. Relevant factors would include time, location and acts under the control of the accused yet to be accomplished.
 Relative proximity may give an act, which might otherwise seem to be mere preparation, the quality of an attempt. Further, an act on its face an act of commission does not lose its quality as the actus reus of an attempt simply because further acts are required, or because a significant period of time may elapse before the completion of the substantive offence.
 To constitute the actus reus of an attempt, the act of an accused need not be the last act before the completion of the substantive offence. To constitute the actus reus of an attempt, an act must be sufficiently proximate to the intended crime to amount to more than mere preparation to commit it. This requirement of proximity, expressed in the divide between preparation and attempt, has to do with the sequence of events leading to the crime that an accused has in mind to commit. To be guilty of an attempt, an accused must have progressed a sufficient distance (beyond mere preparation) down the intended path. An act is proximate if it is the first of a series of similar or related acts intended to result cumulatively in a substantive crime.