RSLogix software does not in and of itself allow you to go backwards. You can't convert the file to version 23 and then go back to version 20. You also can only have one version of the minor revision installed for every major revision. There are some issues with v20 in that regard (20.01 and 20.13 I believe are the exact numbers).
Also, you cannot perform an upload or download until your revision of the Logix/Studio matches the revision on the controller. You NEED to match the version on the controller to do service work.
Now as far as the OP's question. It is simple. You tell them up front you will be more than happy to check it out, and that if they can give you the version up front, no big deal you can make sure you have it before you get there.
I mean, really, if you are trying to do service work in this field, it shouldn't be an insurmountable hurdle. You have a tech connect contract? If not you need to get one and have copies of all the versions ready.
Perhaps they have one. Perhaps you can get a copy from the local distributor. Generally, if possible I try to do an audit before hand and figure out this kind of stuff. If that isn't possible, it isn't a bad idea to tell them that you aren't familiar with the system and you will need someone who is to accompany you. Tell them you are happy to figure the system out yourself if no one is available, but it will add lots of time onto the job. Perhaps they have the proper copies of the software or what you need.
I will throw this in for good measure.
You can probably get the info from RSLinx, or you can try to connect to the IP via web browser to get the info. The fastest way is to just connect (RSLinx won't have issues configuring the driver for other versions). The best and most surefire way is to connect and try to go online with the controller.
If you run into any version before 13.04 you will have to use that version in a windows XP virtual machine, the software will start but will NOT open a project in anything newer than XP.