It's unfortunately very hard to predict how well any particular CPU will overclock. One Xeon E3-1220 might overclock good, and the other might not overclock much at all.
Off the assembly line, each processor, even if they're the same model, have differences that the builders cannot control. It's these differences that cause differences in a CPU's ability to overclock.
That being said, from what I have heard Xeons are generally not really known for high overclocks.
The only really way to see how well a particular CPU you own will overclock is to actually overclock it and see. However, you can always look up and see what kind of overclocks other people have gotten. It might give you an idea what you might expect, but you also might get no where near what someone else got with the same CPU or you might double what they got.
I typed in "overclocking a xeon" into google and the first several links that popped up talked about overclocking an E3-1231 V3, and another with an E3-1220, so you can use a search engine to research what other people get.