How to tell the different ages of pieces I know that LEGO bricks have changed over the years, with regards to moulding pip locations, element i.d. numbers printed inside etc, but I was wondering if anyone knows when these changes happened?  


What year the pip moved from on the side of the brick to on the stud  
What year they started printing the element i.d. on the elements  
What year the heads changed from solid stud to recessed stud  


The reason I ask is because I have started to collect earlier sets from the 1980s and 1990s and these are used sets. I want to know if they are the original parts or if they have been substituted for newer, cleaner parts.
According to Leggodt.nl, the molding pip was moved from the side to the top of a stud beginning about 1974, but as Kramii points out there is no hard cutoff date, as LEGO used molds until they were worn out and included existing stock in sets until it was depleted. So you may find mixed types of bricks in sets between 1974 and 1979. By about 1980, the side-pip bricks were gone. 
The element numbers were printed on the bricks subsequent to the "patent pending" period, which was when many bricks had either "pat. pend." molded underneath, or places where those words had been obscured. Obscured "pat. pend." bricks appeared in sets as late as 1978, which is also when the LEGO logo on the top studs was changed (according to Gary Istok's LEGO guide). I believe this is when the element numbers started to be molded on the underside of the pieces, but again there would be a transition period. By 1980 in the US, all modern pieces were in use.
The hollow-stud minifig head appeared in 1992 and was a change based on product safety requirements (to prevent choking.) According to an article at Gizmodo, LEGO decided to re-introduce the solid-stud heads after reviewing changes in safety laws. According to Peeron.com 1992 was the last year the solid-stud heads were included in sets and they began appearing again in 2006.