We need to be clear about the terminology here.
Crystal - a passive device made from quartz (usually) which exhibits the characteristics of a very high Q tuned LC circuit.
Usually enclosed in a sealed metal can with two pins or wires for connection to a circuit.
Crystal oscillator - either an active device with a crystal and an amplifier, when a DC supply is applied to two of the pins, the amplifier will oscillate at a frequency determined by the crystal.
Usually enclosed in a sealed metal can with four pins or wires to connect to the external circuit.
The four wires are: 0v, +5V, output, the fourth wire is often not connected but may sometimes be used as an enable/disable signal to start/stop the oscillator.
Since the crystal oscillates at a certin frequency - would't it , in a way, act as a bandpass fiter that i could test for?
If the device is a crystal rather than a crystal oscillator then yes you could test the crystal to find its resonant frequency, you will need a signal generator with very fine frequency control, a frequency meter if the sig gen does not have a built-in counter, a suitable test jig to hold the crystal and provide defined circuit conditions, and connections for the test equipment, and some means of measuring the output voltage from the test jig, an oscilloscope or RF voltmeter would do.
i can only power and test the crystal
You cannot "power" a crystal, a crystal oscillator yes, but not a crystal.
JimB