Anyway depending on the brand of silicone used to build it, the kits technical data sheets vary in information what to do before taking the tank in use as aqaurium.
Almost all advice to leave the tank an X periode flooded and still flush it several times after that. And i don't think that aquarium builders keep this in respect, after all time is money and it takes space and time to perform these action. So i assume they build the tank cure it given time, fill it up and actualy my own experience in tank building says, if it doesn't leak within an hour it probably never will if installed correctly. Than drain it and store it..
Flushing and cleaning is the new owners responsibility..
Than beter safe than sorry, try to find out its builder and ask what their procedure is and which brand of kit is used. If that is to no avail, than flood and drain the tank a few times at least 2 x with 24 hours in between, before you are going to install it permanently and set it up.
The first tiem you can clean it as ian_m says with what ever you like, than start the flooding and draining cycle and you'll flush it clean..
My personal experience to clean a brandnew tank especialy if it still has kit residu and fingerprints on the glass is using a soft cloth damped with dish washing soap.
Then flush it a few times and wipe the inside panels with a clean cloth for a bling bling tank..
After that is the so called cycle periode with the abundand water changes, after 6 weeks what ever tiny bit was left will be long gone..