Is communion service at the rail vs. in the pews a function of denomination or church size? In the small baptist churches I've been a part of, (and the LDS service I attended), the communion elements are served to worshippers in the pews. In my Episcopal church, everybody comes up to the rail.  I'm wondering a bit about how this variation in practice arose (Anyone know which came first?) and if there is a way of predicting how any given church will practice it.
In Catholic churches, Communion is required by the Holy See to be distributed by a minister (priest, bishop, deacon, etc) and may only be done so by a lay person if there are not enough ministers to do the job. This, and the requirements about how the congregants receive the Eucharist from the minister, suggests that in Catholic churches, altar distribution is the way it goes. This is one of the many Catholic practices that originated in the Catholic Churches need to control its congregants. The ministers have control over whether you receive the sacrament of the Eucharist.

Episcopal/Anglican churches also practice this distribution by ministers, although I'm not certain about requirements as I know less about the Anglican church hierarchy. Lutheran churches do too, sometimes referring to it as The Sacrament of the Altar. Both of these come from their close ties to Catholicism back in the day.

In the various other Protestant persuasions, the desire to draw away from the rules and rituals of the more orthodox denominations and the belief that God connects directly with all people, not just through the ministers, lead to a significant de-formalization of the communion rite. Also, since most Protestants do not hold the that communion is a sacrament, but rather a remembrance of Christ's death for Christians on the cross, the formal ritual became unnecessary. 

Also because of the lower importance that Protestants place on the Eucharist, it is not celebrated weekly at all churches, including Lutheran and Anglican churches. Catholics, however, are required to take communion weekly.

As to whether you can tell, at Protestant churches, there is not a sweeping generalization to be made based on denomination or size. That said, you could probably tell by when you walk in if you see a communion table set up with those communion cup-holders stacked on top of each other or if you see somewhere to kneel at the front of the church.