From what I have heard explained, they like to expect that it is, however,
they have a set of questions that they ask to fill in the blanks.
Some of the problems associated with Protestant baptism are:
The minister had no intention of doing what the Church teaches, but only giving
the recipient a kind of initiation or membership in the particular congregation
that he represents, like paying dues for a club and getting on the roll call.
The words used may not have been proper words but something more
"interesting" like I baptize you in the persons of the Father and of the Son and
of the Holy Sanctifier.
The water may not have been placed on the skin of the forehead but only on
the hair. It may not have been an amount sufficient to run across the skin.
Perhaps the recipient was old enough to know, and did not want to be
baptized.
In such cases, a conditional re-baptism could be arranged, to remove any doubt.