How do you go about doing I never have tested coax before and should it show 50 ohms
You can't measure characteristic impedance with a multimeter unless getting into some complex measuring and math formulas. Not really necessary for scanner purposes as 50 or 75 ohm doesn't really matter for receiving only. You can only really check for continuity and shorts. Not sure exactly how familiar you are with coaxial cable I guess, but essentially it's two separate wires running concurrently, or together. The inner core wire, which is wrapped with an insulator such as plastic or foam, then the braided shielding around that. Then it's wrapped in a rubber jacket. So basically you are concerned with two wires. Continuity means you have a good wire from one end to the other, for both the inner core wire, and the shielding at their respective ends. You WANT continuity, or a continuous wire between the center wire at one end, and the center wire at the other end. Then, you WANT continuity between the shielding at one end and the shielding at the other end. You DO NOT WANT continuity between the core wire, and the shielding. That means you have a short.
To test this with a multimeter, you turn the dial to the continuity function. It should be a symbol like the one attached. It might also have like a little speaker symbol too because it should beep or tone when the two leads are touched together. Test that it does that to make sure your meter is working right. Remeber, it's testing for a closed or "continuous" circuit path. Now, touch one lead to the center wire of one end of your cable, then touch the other lead to the center wire on the other end of your cable. If you have good continuity, it should beep. Now, touch one lead to the outer metal part of the connector, which is crimped to shielding of the cable, and the other lead to the outer metal connector on the other end of the cable. You SHOULD have continuity. Now, to test for short, touch one lead to the center wire of the cable, and the other lead to the metal outer connector. You should NOT have continuity and it should not be beeping. If it is, you have a short between the outer shielding and the center conductor. Most likely a rogue sliver of the braided shield snuck past the crimper and is touching the center conductor. Just need to cut off and put a new end on.