If your not sure then definitely backup the eeprom and put a copy on your PC in case anything goes wrong. If you have Krayzies Ndure v1.0-v1.1.1 installed then you will get a recovery dash if you try to wipe it, if you have SID's Knave 4.0-4.5 then you will get "Error 22". All other softmods can be wiped fine (with the exception of Ndure via XBOXHDM). So having the eeprom is the best bet. If you have Krayzies already and do boot to the recovery dash after using CASH that can be fixed still using CASH, "Error 22" or an errored box from Ndure via XBOXHDM will have to be fixed using the eeprom and XBOXHDM (Error 22 can be fixed via AID also).
One good way to find out is try asking the guy you bought it from, I'm sure they wouldn't mind telling you what softmod it is running. I'm also guessing since you bought it softmodded you don't have the exploit tools (a memcard, AR, or USB device and one of the 3 exploitable games), those are very handy to have, but not 100% needed if you have the eeprom (since you can rebuild with XBOXHDM, but that's not always fun).
I would say if you feel comfortable using XBOXHDM and feel you can rebuild the drive if anything goes wrong then feel free to use CASH, but if you don't feel comfortable doing that and the box is working I'd leave it be for now.
Hopefully in a future version of CASH i can design and implement a softmod detection tool that will detect the softmod used, would be extremely helpful in situtations like this.