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When can particles be distinguished?
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Protons are said to be ''identical particles'' - which has a precise 
formal definition that translates to the informal meaning that they 
don't have an identity in themselves but get them only through their 
environment. 

So one can identify a proton by ''the proton passing at time t near 
position x'', or ''the proton just detected by the Geiger counter'' 
but it makes no sense to say that ''this proton is the same as that 
one'' unless you know (or assume) that it followed a predictable 
(approximate) trajectory and you know (or assume) in addition that no 
other proton could have taken its place.

The same holds for all elementary particles, but also for composite 
particles if they are small enough. Particles begin to become 
distinguishable when they either are confined to a lattice (such as 
atoms in a crystal; then they are distinguishable by their position), 
or when they have so much internal structure (e.g., macromolecules) 
that the structure of any two is distinct enough to make them 
experimentally distinguishable by measuring their different internal 
structure.