There are a few ways to tell if your car speakers are blown:

1. Listen for distortion: If you hear distortion or static coming from the speakers, even when the volume is turned down, it could be a sign that the speakers are blown.
2. Check for physical damage: Look for any visible signs of damage, such as tears or holes in the speaker cone.
3. Test the speakers: Try playing a variety of songs at different volumes to see if the speakers are producing sound as they should. If the sound is distorted or there is little to no output, the speakers may be blown.
4. Use a multimeter: You can use a multimeter to test the resistance of the speakers. If the resistance is very high or infinite, it could be a sign that the speakers are blown.

It's also possible that the problem could be with the head unit, wiring, or amplifier, rather than the speakers themselves. If you're not sure what the issue is, it may be best to take your car to a mechanic or a car audio specialist for further diagnosis.