The notification symbol varies somewhat.
In the notification bar upper right, it will say 2G for, obviously, 2G, or E for Edge, which is also 2G.
3G, for again obviously 3G, or H for HSPA, which is 3G.
If I remember correctly, you could see H or H+ for HSPA+ which is kinda sorta 4G.
Finally, LTE indicates 4G LTE, of course.