It's usually obvious by the context, but almost all 'voltages' measured will be DC - AC is in a tiny minority. This doesn't refer to 'signals' which are a different thing entirely.
If it's above about 90V then you can use a neon lamp and a suitable series resistor, the cathode will always glow brightest and if its AC then both electrodes will glow.
If the ac is not of high frequency then we can see using a multimeter.Voltage will be continuously fluctuating.In the same way logic probe can also be used.