I am looking at a base station and want to test it before purchasing can I use a magnetic mobile antenna to see if it transmits and receives without damaging it?
Magnetic mobile antenna is supposed to be mounted on the roof of vehicle, thus metallic roof serves as ground plane.
If you want to use it as base station antenna, off the vehicle you have to provide similar ground plane. As it is magnetic antenna it probably does not have option to attach radials so you have to set it up on large enough sheet of metal.
For VHF and UHF, side of PC case is good enough. For CB I guess it should be larger.
Anyways, you should test transmitting on low power first (less than 5 watts) and with SWR meter. If everything is OK then you may switch to higher power (but not higher than what is antenna rated for)
If you are using a magnetically-mounted antenna that is designed for mobile use, then ideally you need to put it on as large a sheet of metal as you can find. This is because a mobile antenna of this type is (as mentioned in another answer) designed to use the roof of a vehicle as a groundplane.
The President Washington should only transmit about 4 watts of power (Google is your friend), so any small antenna should do for testing purposes. Do be aware that some people do like to increase the power out to 15-20W, which puts the radio out of spec. If this is the case, you would be well advised to have the radio realigned - which won't hurt anyway, as these sets are getting long in the tooth now.
Ideally though, you would want to take a power meter (a regular power/SWR meter would do just fine) and a small dummy load. Plug the dummy load into the antenna socket of the power meter, and plug the power meter into the transmitter. That will tell you how much power you are transmitting.
For testing how well it receives, any antenna will do really. A mag-mount on a balcony railing will be fine to see if the radio's receiver is working at all.
In general, as long as the antenna is reasonably sized for the transmitting frequency it should be fine. In other words, if you are transmitting on 30 MHZ (10 meter band) and you try to use a 12" mag mount, you can either damage the radio at worst, or best case the radio clamps down the output and you won't get any signal out.
Keep in mind that you will probably not get great performance out of it, but it should at least be good enough to test with another close by radio to make sure that the base station is actually working.
using a mobile mag mount antenna works great. I've used this set-up many times. Wilson 1000 connected with a barrel to 50' lenght of coax. There will be some line loss so 50-100 rf amp fixes that. have fun. only concern which should always be is low swr. nothing above 1:5