As far as I know there is no particular standard or presiding body in most cases that would certify someone as an expert. Also, some experts are more expert that others - thus we have both sides presenting their own experts in the same fields with differing opinions.
The judge does not usually certify someone as an expert witness. (I have testified as an expert witness several times.) It is up to the attorney that is paying the tab for the expert witness to have the witness establish their expertise by stating education, practice in field, inventions, patents, years practiced, etc… The common counter move by opposing counsel is for their side to interrupt the expert and tell the judge/court that they recognize this witness as an expert in this particular area and acquiesce to same stealing the opportunity to impress the jury with all the stories of all the wonderful stuff this 'expert' has done and why they should be considered an expert.
There is absolutely a standard.
(In a small nutshell) The witness must demonstrate that they have the necessary qualifications to present their opinion, including having education/training and practical experience in the area and the opinion must be based in accepted practice.
If the judge doesn't believe the witness meets the qualifications then they are not an "expert" and their testimony has no relevance. It is usually up to the party opposing the witness to raise issues of lack of qualification.
they would never smoke crack in Cupertino but they do have some kush that would rock your sh*t, that patent where they "pour" information from one device to another still boggles my mind..