Silver is hallmarked so look for the stamped markings which will tell you an age, maker and assay office - i.e. is it from London, Birmingham etc.
EPNS feels heavy in the hand and is more likely to be the material - it'll have this stamped on it somewhere as suggested. Sheffield plate is the thing you are probably be thinking of - this is steel with a 'coating' over it to improve the finish.
If it is any quality all makers stamp their name on it - if there is no makers name, have a trawl through some general auction houses in your area - they are most likely to have cutlery in job lots from huse clearences and the like and this is probably the best bet to make up the set you have.
It might have a number like 325 on it. This is silver plate. Are than handles of ivory or an imitation ivory like a bakelite plastic? If they are of a certain age, a decent antiques dealer can advise about matching, but I would say the auctioneers near you will be best.
Hope this helps..