If the light is ugly as far as color casts, then you use an 18% gray card and measure the light using the cameras measure mode in White Balance. If it is casting shadows then you have the on board flash, which I know to some its use is an anathema, but if you need it to offset the harsh lighting then it is better that a screwed shot. If possible get someone to stand in the area and see what effects the lighting has, then try your flash and adjust it so you get the proper exposure. This means setting the Flash to Manual and choosing an put put. You only want enough to offset the lights, and get a good exposure. Also use the Redeye for the flash as well. That is the one problem with the on board flash.