With each suite of P&IDs for a particular installation, one of the sheets needs to be a set of typical symbols for the equipment depicted. Otherwise the value of the rest of the drawings is somewhat reduced.

Ask for the legend sheet for the drawing series, or the standard symbols sheet, from the drawing office that produced them.

I might be recalling this incorrectly, but I think there is an ISA (International Standards Association) that has a publication on standard notations for P&ID's. Not everyone follows this, so PW's suggestion is a good way to start. On drawings, look for a "cover sheet" of abbreviations and symbols, etc.

The conventions differ among various regions of the world, but usually are not very difficult to figure out and reconcile.

Look at some of these:

http://www.pids-a-drafters-perspective.com/index_PIDSymbols.htm

http://www.demet.ufmg.br/grad/disciplinas/eqm033/n04pfd.pdf

http://www.ullimanschutte.com/documents/projects/Fort%20Detrick%20WWTP%20Repair/Addenda/Addendum%203/Revised%20Drawings/D-001%20PROCESS%20SYSTEMS%20P&ID%20LEGEND%20AND%20SYMBOLS.pdf

http://www.scribd.com/doc/4035526/Sample-PID-Symbols-and-Legends

Or

Google "P&ID Symbols"

Your most authoritative source would be ISA standards:

http://www.isa.org/Template.cfm?Section=Standards8&Template=/customsource/isa/Standards/StandardsListing.cfm

These, however, are not free. There is a chapter in my online e-book discussing instrumentation diagrams, which you might find helpful (Chapter 7, Instrumentation Documents):

http://www.openbookproject.net/books/socratic/sinst/book/liii.pdf