How to identify iPhone 3G vs 3GS and 4 vs 4S

iPhone 3G

iPhone 3G in white color has "iPhone" text clearly readable and text is not same as background color on its back side. In black color, "iPhone" text opacity is low and doesn't seem clear and white.

iPhone 3GS 

Black 3GS contains "iPhone" text clear white in high opacity. White 3GS contains "iPhone" text as same as background color in very high opacity.

iPhone 4 and 4S

                                                             4S                                        4

More about.

Model	Year	Model	Storage
iPhone 4S	2011	A1387	16/32/64 GB
iPhone 4 (CDMA)	2011	A1349	8/16 /32 GB
iPhone 4 (GSM)	2010	A1332	8/16/32 GB
iPhone 3GS (Made in China)	2009	A1325	8/16/32 GB
iPhone 3GS	2009	A1303	8/16 /32 GB
iPhone 3G (Made in China)	2009	A1324	8 /16 GB
iPhone 3G	2008	A1241	8 /16 GB
iPhone	2007	A1203	4/8/16 GB

Note: Look at your iPhone's back case to find model no: