Eating with silvered cutlery
It's very likely to that the base metal is some kind of alloy, and not simply lead, but if you're in doubt then you should probably stop using them.

Unless solid silver, most within the last 100+ years made in the US, UK and Germany will likely be some mixture of copper / nickel / tin. Some might have pewter detail and decoration which might contain lead, but typically not on the tines / blade / etc. Pewter as a base for silver-plating saw its real beginning after antimony was predominately used to make it, but you can only really guess unless the company is still around and confirms it, or you have it tested. Most which is still in use today would likely be made of a nickel alloy under the plating. 

If you're at all in doubt, or get an odd taste, you can send them out to a plating shop, or just polish and display them and get something you know to be safe. Odd metal 'tangy' tastes are probably coming not from lead, but copper in the alloy, but always go on the safe side here. 

You can, as folks noted in comments, have them tested. However, unless you're really keen on their continued use due to monetary or sentimental value, the cost / reward ratio could get lopsided quickly. 

If they have a maker's mark (or other markings), you might be able to get some more information from an antique dealer, which could at least get you an appraisal in addition to knowing more about their makeup.