If they're cartridge bearings, there will be a code on them.
If they're loose: http://www.parktool.com/product/spoke-bearing-and-cotter-gauge-sbc-1
This is my problem - someone's removed the old cartridges...
Show us a picture of the disc/hub and someone might be able to give you an educated guess at which bearing. Otherwise, you may need to buy some vernier calipers.
@Miitch started
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If they're cartridge bearings, there will be a code on them.
If they're loose: http://www.parktool.com/product/spoke-bearing-and-cotter-gauge-sbc-1