How would you detect poles with a Lego NXT?

[Crackass]

So basically I am doing the sumobot challenge where you have to build a robot that has a max length and width of 20 cm each. The arena of the competition is a hexagon with white borders and has 6 poles. The rules and everything is online and is pretty self-explanatory and we are also allowed to have 1 extra sensor than what is in a standard nxt set which is 1 of every sensor in case you were wondering (so we could have 2 light sensors instead of 1). I was wondering how we would detect the poles and and avoid them without interfering with the actual pushing off other people (so not as simple as a touch sensor in the front since other bots would push that too. Someone told me there is multiple ways to do it. I don't really know any efficient ones. Any ideas? Sorry if I posted in the wrong area I am new.

[Auranapse]

Is there a picture of the arena?

[Hrafn]

How big are the poles relative to the robots and the arena, and are they taller than the robots? Are there any height limits on the robots?