well, good for you!
now, generally, if the timing belt is done by a Toy or Lexus dealer, they will/should put a sticker under the hood somewhere. Of course, they may not. An independent would likely not do that either. Plus having the sticker does not necessarily mean it's actually been done, although likely... How's that for certainties..?
So basically, you may have to depend on copy of bills, original parts in the trunk, indication of work on the engine (bolts, gaskets etc), your trust of the seller etc. If it's an LX it may be in the Lexus system which may be easy to look up. Probably not for a regular Toyota. Given the high price of the job, a private party would likely have the records, but a dealer perhaps having bought it at auction, well.... You could have the dealer give a bunch of phone calls and try to find out.
Sadly, it may not be a fruitful search, and if so you should -for value purposes- conservatively assume it's not been done. The bummer is that you have then then to figure out whether you should do it yourself (perhaps all over again unnecessarily, but OTOH if you don't do it, it may -highly unlikely of course but possible- explode in the near future, who knows...). I don't recall if there was ever a first hand account here of a timing belt failing but it's certainly not a common problem. The downside is that not doing it is a major gamble as you could conceivably ruin your engine in a major way.
If you want to use the timing belt as an argument to lower the cost, try and go have a look under the hood yourself before discussing this more and having a sticker magically appear...
The price for the job varies surprisingly widely depending on who does it, but IIRC is usually somewhere between $1000 and $2000 at a Toy dealer with the desirable side jobs, so not trivial for a $12K vehicle.
Perhaps somebody here having done it himself may think of a clever way to ascertain this somewhat more definitively (non-OEM color goop or gasket or something?).
Basically, it sucks not to know for sure.
Of course, if the price is really very good (seems good but IDK for sure) or already reflect that uncertainty, well, then all is well at least $$ wise.
) 