Here's what you can do, if you don't have a scope and a generator. Buy a Cardas test record and a digital multimeter. They are both good tools for any audiophile to have and can be bought for not much moola. The Cardas has a band that plays a 1kHz tone at 0db. (DON'T play it over your spkrs, unless the volume is turned off or way down.) Start with a load resistor that is way higher than what's likely to be "critical". While playing the test tone, measure AC voltage at the output of your phono stage and record it. Now reduce the value of the resistor stepwise, and keep measuring ACV at the output of the phono stage, until you first detect a drop in voltage. Go back up from there by one step and you've probably got a pretty good approximation of "critical damping".
