If one is a dogmatic sede, wouldn't it make sense for them to deny communion to those who recognize and follow someone who, in their views, is an anti-pope?

Similarly, I would assume those priests who hold sede-ism as a mere opinion, wouldn't be in a position to deny communion to one who recognized BXVI as he admits he could be wrong.

Well, I'm not certain what "using sacraments as weapons" means. If the denial of communion is used as a way to stigmatize certain individuals one doesn't like in the congregation, that would be evil. If the priest is a dogmatic sede and denies all sedeplentists communion, he would at least be consistent.

Trinity, I'm no expert, by far, but I will try to answer your question as honestly as I can.
'Using the sacraments as weapons' IS quite different from refusing the sacraments to unbelievers.  First off I want to state that I believe that there are priests who do use the sacraments as weapons & I'm not going to state specific examples.  Secondly, sedevacantism is only a personal opinion & not a doctrine of the faith.  Some of us believe in it, some do not, just as Fatima & Lourdes are not doctrines & some of us believe in such apparitions & some do not & no one can or should question us about them.  Therefore none of these things should be a basis for whether we may receive the sacraments.  Priests who do are usurping the authority of the Pope.  This is splintering Catholics & should NOT be happening.  We pray for unity at Mass & then go out & divide into groups - for or against sedevacantism!!  This is utter nonsense & needs to stop.
Refusing the sacraments to unbelievers is quite different as you now see because an unbeliever is denying a doctrine of the faith & only those who believe can participate in any sacraments.

Your second question is a little harder to answer & I'm at a loss to answer except to say that we need to be more vigilant & not accept people at face value.  There's a lot of fakes out there & they flourish because many Catholics are naive & too trusting when they see or talk to someone who SEEMS pious & knowing.  Just remember - an actor can go thro the actions of a Mass & just because someone does the Mass perfectly or someone can appear pious & knowledgeable, doesn't necessarily mean that they are 'for real'.  

I guess the only thing I can offer is to be on guard & come to the realization that we can't take on the whole problem - we only need to strive very hard to get ourselves & family to heaven.    

This might help:

Article II. The Recipient of Holy Communion.

696. Every baptized person not barred by law can and must be admitted to holy Communion. (Canon 853.)

698. Notoriously unworthy Catholics, such as those excommunicated, interdicted, and of public bad repute, must not be admitted to holy Communion until after their repentance and amendment is known, and satisfaction has been made for public scandal. Occult sinners who secretly ask for holy Communion should be refused by the priest if he knows that they have not amended; if, however, they ask publicly and the priest cannot pass them over without scandal, he may give them holy Communion. (Canon 855.)

So it all depends on the priest's motives, Correct?

Where do you get that? Please read the above from a well-known and respected commentary on Canon Law. It seems that the refusal of the sacraments is quite limited by the Church. The faithful have a right to the Sacraments. The priest has a duty to provide them to those who ask and are not barred by law, "excommunicated, interdicted, and of public bad repute."

Because it is the priest who decides who is barred and who is not.

Except he must base his decision on what the Church instructs.

696. Every baptized person not barred by law can and must be admitted to holy Communion. (Canon 853.)

roscoe, known un-Catholic means that the priest knows a particular person at the communion rail is indeed a non-Catholic.  President Clinton a non-Catholic received "communion" in the novus ordo church, sorry you missed that piece of news.  

A priest can not judge the soul of a person when he is at the communion rail, because a person can be in the state of grace after confession, and it could be that a person who just walked out of the confessional might not be in the state of grace.  A priest must assume that every person who goes to receive communion is worthy.  He can not judge at the communion rail because receiving the Eucharist and going to Confession are two different sacraments.