The failing GPUs in these units are all subject to failure in a timeframe most would call premature. This is because the the cooling system and the generation of and the vulnerability to heat of the AMD chips are mismatched. All official Apple repairs were nothing more than swapping out the entire logic board and replacing it with the exact same thing, spec wise. Above normal room temperature and really stressing the GPU with 3D rendering while a second monitor is plugged in (getting even more heat from the thunderbolt chips) is a surefire way to further shorten the lifetime of the AMD chips.
Investing into a used machine from 2011 with AMD chips inside is a bad idea unless it is really cheap. That is because the issues can be indeed latent in the early stages. The problem starts with different symptoms and in different stages, some of them might be gone for a while, especially if everything else is unplugged, the fan speed upped to ~2500–3000 (almost inaudible for most people over 19 or so), air conditioned room, etc. [To use a AMD chip in this generation of MBPs the chip itself has to replaced with a more robust, later, cooler revision and ideally the cooling system improved (better thermal compound better applied and additional holes for the airflow)] For an example of latent issues in the early stages see this post.
But most frustrating would be a common practice popularised on youtube and other sides: reflowing/baking the logic board. That is a failed AMD chip is heated to 120–200 degrees Celsius and thereafter shown off as a re-animated corpse. But only for a short time compared to the time it was running before with no apparent problems. This procedure has a kind of half-life until the problems come back, and worse that time, breaking even sooner than before and eventually failing completely with no amount of baking doing any good any more.
That means that a self-sourced OS is of only limited value to protect against a rogue seller. A GPU treated in this way is dying soon but likely not in the time it usually takes to make a quick test of basic function in a typical sales situation. This is a latent problem in my definition.
Nevertheless it is not really possible to protect yourself from a rebaked chip. But it is possible – in a limited way – to guard against someone showing off a GPU-switching Mac with only intel GPU active (like certain hacks):
Boot your own OS. That is: macOS from an external drive. Then go into Apple Menu>About This Mac>System Report and check what is listed there. Since your own OS will contain the drivers but not the hacks: Manipulated to iGPU-only Macs will only list the IntelHD3000 chip there. (But also very likely hang on boot with your stick/drive) If you see trouble on boot in this configuration: reset the SMC/PRAM/NVRAM, reboot again and if there are immediately visible artefacts: then confront the seller!
Boot your own OS. That is: try a LiveLinux like Ubuntu. Problems already manifest show up quicker there since even the basic acceleration there (without the proprietary drvers) stress the chip too much. Also check the hardware from your chosen Linux distribution.