a vent should look like a vent either in walls or floor . a mesh or grill will usualy be on the front of this but not always. and sometimes will have a sliding plate to open / close of the vent.
you have double glassing , some come with vents in others without , again these look like vents if your windows have them.
i would check the links i have supplied as they tell you when a vent is required , for instance as has been stated a 7kw or above fire would require a vent yet below that no vent is deamed needed.
if you get your fire serviced annualy and have no warning from your gas service engineer then i imagine its just a case of having the heat to high therefore making it seem like less air is in room.
However you can check your fire KW level by doing a search regarding your particular fire online and finding its KW level that way. taking the information of the fire itself or relevent paperwork " if you have any "
hot air is dryer and seems thinner some people find the air is too dry when they use their gas fire and often put a little pot/bowl of water under the fire to add some humidity to the room
if your property is rented , and your fire is above the 7KW level then your landlord would be able to enlighten you as to where the vent is.
http://en.wikipedia.o...on_monoxide_poisoning
that is useful info to know regarding this.