Hi,
With the meter on an AC scale, connect a capacitor in series with the meter and take a reading. The meter can only read AC then so if the signal has AC content it will produce a reading, but if pure DC then it will read zero.
The reason for using the capacitor is because some meters will read voltage on the AC range even when there is just a DC voltage. The cap ensures that only AC can get through.
Of course you have to use a cap that is appropriate for the expected voltage, and it must be non polarized and preferably rated for AC operation. I keep a 2uf 200v non polarized mylar cap around for such measurements.
Note that you'll see a reading if there is any AC component even if it is small, but then the reading will also be small. Thus the ripply from a half wave rectifier for example will show up a little but it should be low.