How to measure cutoff frequency of a low pass audio filter?
As Andy mentions in a comment, you need a signal source that you can tune to make a frequency that you measure the filter with.

Luckily you say it is an Audio filter, so you can use an audio source.

Find a program that you can use in the freeware (or sample-ware) world to make signals around the cut-off frequency.

Use your scope to measure the signal going into the filter and going out. The scope can also probably even do some maths (since it's a digital one) and show "Input" - "Output" for you to easily then calculate the factor.

Since you designed the filter you should probably know about the log() scale of Decibels, but since I'm in a decent-ish mood today I'll continue to explain that the "cut-off" frequency is the 3dB point for many filter designs.

The 3dB point is where the power is reduced by a factor of two, but that means for the voltage that it should be a factor of sqrt(2) =~ 1.41 lower. Half the voltage output would be 6dB.

This website has a nice table to use if you don't feel like maths. 

Many digital scopes can also (or have a purchasable upgrade for it) calculate the dB change in voltage. All you'd need to do is tweak the sine-wave output by the sound card until it says "-3dB". These digital toys have really taken the art out of it :-).

In fact, if it has "infinite persistence" you can make your soundcard sweep the frequency over 10seconds and then set the scope to a timebase that gets 10 seconds for a full screen and you can get a nice 2-line frequency plot and just use a ruler to instantly find a 3dB, 6dB or 20dB point.