Actually, the method they use to find out the age of the fossils is by the layer of Earth its found in. And guess how they decide which age each layer declares what's found in it? By the fossils in them...
Ugh, I studied Paleoanthropology and I lose this thread.
The person who told you this was either a creationist or a child.
How about getting information from sources that actually, you know, study these things as their discipline?
The problem with Creationism is it comes from a reactionary place trying to disprove something posited by a method which simply arose out of a discipline of truth-seeking.Science is limited by what can be proven by evidence, if Creationism had evidence for its claims, Science would have no problems updating its theories. Creationism is always going to be more biased and less informed because its principle is to reject, by any means, anything contrary to its dogma. In other words, its not interested in empirical truth.
There are dozen's of techniques used to date fossils including which stratum they are found (which is not the way the geological age is determined), but also using many other kinds of methods, some environment dependent and some not, depending on the place you find the fossils and how old they might be.
Scientists will compare the findings of as MANY techniques as they can against each other. If they all point to approximately the same time range, guess what? The fossil is probably that age.
Its sort of offensive and extremely ignorant, and intellectually dishonest to simply post something that basically implies scientist are idiots, especially when you haven't even superficially delved into the topic.