Measure the inside width of the rim (in mm) and consult this chart: http://sheldonbrown.com/tire-sizing.html#width (or the width may be printed on the wheel's label). 1" = ~25mm, 1.4" = ~35mm.

Newer MTBs seem to be coming with narrower rims than they used to. You can't necessarily base anything off what the current tire's width is. My MTB's rims are 17mm (as labeled), so it should work with either.

If your rim is wider than 17mm--according to the chart--you should use the 1.4" tires, but the chart may err on the side of caution. According to the chart, my 2.1" (~54mm) tires are well beyond the recommended maximum width of my 17mm rims.