1. Try to determine the model before you go so if the machine is not threaded up at least you can a.) look up how to thread it, b.) know the needle size and how to properly seat the needle.
2. Determine, before hand, if there if the bobbin case and at least one bobbin is available for testing.
3. Take the proper needle with you.
4. Take thread with you.
5. Take fabric with you.
6. If the machine has none of the above (and you didn't bring any of the above with you) , hand rotate it to get a feel if you 'think' it will work.
7. If the machine has none of the above available (and you didn't bring any of the above with you), use that as a bargaining point if you 'feel lucky'.