In a nutshell, manufacturer's spec's will determine polarity. With that said, most electrodes, i.e. the '18' series (7018, 8018, etc.) are electrode positive. With wire feed welding, wires requiring gas shielding (solid wire, flux core with gas, etc.) are electrode (gun) positive. Most self shielded wires are electrode negative. There are a few fast fill self shielded wires that are run electrode positive, but again, that depends on the manufacturing process, and can be determined by the specification sheet.

Most cast iron electrodes are made to be run DCEN (Direct Current Electrode Negative). Most SMAW (Shielded Metal Arc Welding) electrodes for mild steel or structural steel, are run DCEP. The P = positive. As a general rule stainless steel SMAW electrodes in the 300 series are DCEP.
As stated in an other post, Most gasless wires FCAW (Flux Cored Arc Welding) are run DCEN.

GMAW (Gas Metal Arc Welding) and FCAW That use gas will usually be run DCEP.
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I believe you'll see polarity at DC Reverse (+) and Straight(-) or AC when checking for electrodes.