Is there a reliable way to measure how difficult it is to be admitted to a PhD program?
I am not aware of any absolute measure for this. However, you can have a general idea by: 


Some schools provide last year(s) statistics about their programs
(usually in the prospective student section). For example, program A
has attracted X number of applications and Y applications were
accepted.
Also, some programs have a fix limit (i.e. due to number of equipments available)  for number of applications to be accepted. 
Sometimes online admission results like the one in Grad Cafe
give you a sense on how strict the school is.