That's easy: Baptize in the Name of the Father, the Son and the Holy Ghost, by triple immersion. :)
The real problem is when there is a good possibility that a person would be re-baptized or "double bpatized" through negligence.
The bishop is then probably under his own obligation to investigate the parish practices and determine how it is done, and make a decision based on that.
But I am sure, the bishop will wish to remove all doubt before subjecting someone to another Baptism.
Very good points. To which I would add just a couple things. Some sects baptize by immersion and use Trinitarian language but have no sacramental intent. Baptists are generally an excellent example of this. Their baptisms are void and empty (IMO). Also in Orthodoxy a priest is the normative minister of baptism. Layman can baptize (as is true in the RCC) in emergencies. But a lay baptism is almost always followed by a corrective (conditional) baptism done by a priest. Church canons do not allow anyone baptized by a layman to enter into Holy Orders. Also unlike in the RCC (which oddly accepts it) in Orthodoxy those not baptized themselves can not under any circumstances baptize anyone. To perform the Holy Mysteries one must be connected to the Mystical Body of Christ sacramentally. This raises all kinds of questions about heterodox baptisms, especially Protestants. In the modern day and age even in the so called confessional churches we don't know what they are doing anymore or what they think they are doing. Look at the Episcopalians. This is one reason why I disagree with the OCA's decision to follow the Russian custom of accepting some Protestant baptisms. I think all Protestant baptisms should be presumed void and beyond the reach of Chrismation, unless you know for certain the manner of the baptism, the intent of the one performing it, and if the baptizing person (Protestants do not have real clergy) was him/herself baptized. Given the extreme unlikelihood of satisfying these points I think its best to just baptize Protestant converts unless there is an unusual reason for an exception.