The required proof includes:
1) testimony by an investigator identifying the item as that which the investigator discovered and took;

2) testimony by that investigator that there was no tampering with the item while it was in his/her custody;

3) testimony regarding delivery of the item to the second person who had custody of the item;

4) possibly similar testimony by the second and each subsequent person who had custody of the item until the time of its presentation in court.  Where the item has been submitted to a laboratory for analysis, proof of the chain of custody should ideally include: testimony from the person who took the item (or specimen) to the laboratory; proof of the method of reception and storage at the laboratory prior to and after analysis; up to the time of trial. Arnold, N.J. Practice, Criminal Procedure,
Sec. 1034.

The most difficult aspect of the proof specified above is usually the identification of the evidence by the investigator who discovered it. This difficulty arises because of the frequent failure to properly "mark" the item. "Marking" means the placing by the investigator of at least his/her initials on the item. Unfortunately, sometimes items are "marked" by affixing an evidence tag to the item with a string. The investigator then puts his/her initials on the tag. When the string breaks and the tag is lost it may then be impossible for the investigator to identify the item as being the item that was discovered. Arnold, N.J. Practice, Criminal Procedure, volume 32, Section 1034.

Many excellent texts, one such text is the Handbook of Forensic Science, published by the Federal Bureau of Investigation provide information on the proper "marking" of various types of evidence, and they should be studied by investigators having responsibility for the collection of physical evidence. But the basic rule is as follows: The item should be "marked" by the investigator placing his/her initials, date, and the case number on the item itself. Metallic surfaces should be so "marked"
with a machinist's scribe. Liquids, soils and small fragments should be placed in a suitable container and sealed. The container should be "marked" by scribing the same information on the container, or by using some other permanent form of marking material on the container. Arnold, N.J. Practice, Criminal Procedure, Volume 32, Section 1034.

With respect to avoiding a claim of substitution of another item for that seized or a claim that the item has been tampered with, the problems of proof can be minimized by designating one investigator as the custodian of all the physical evidence in a given investigation.

All investigators who recover physical evidence must turn that evidence over to the custodian, who is then responsible for the evidence from that time forward until trial. Where evidence must be submitted to a laboratory, the custodian delivers that evidence to the laboratory, and obtains a receipt from the laboratory. When the laboratory has completed its examination, it is the custodian who returns to the laboratory, receives the remaining specimen1 from the laboratory, and retains custody of the specimen and brings it to court for trial. By following this procedure, all the physical evidence can be introduced by
calling the various investigators who recovered and marked each item of physical evidence, the custodian, and the laboratory specialist who examined the evidence. (The laboratory specialist testifies not only with respect to the laboratory examination, the specialist's findings and opinion, but also as to the method of reception and storage at the laboratory prior to and after analysis.) Arnold, N.J. Practice, Criminal Procedure, Volume 32, Section 1034.

The identification of evidence and chain of evidence rules require that the proponent of the evidence show that the evidence has not been tampered with, and that there has not been any irregularity which altered its probative value. State v. Roszkowski, 129 N.J. Super. 315, 323 A2d 531 (App.Div. 1974).