I have been riding a '98 YZ400 for about 3 years now. Mine was also brought not running and with a 'claimed' recent rebuild. The owner did however have the rebuild receipts and they had been done by a reputable shop.
I got the bike running and it has been sweet ever since. If you scroll through this forum you will see these are good reliable bikes.
It will most likely take some effort to get going, especially if the fuel has been left in the carburettor. You will have to strip the carb and take good care with cleaning out all the passageways, some are very small and may be blocked even though they may look clean. ALso take note of the sticky's at the top of this forum, there is a thread there with video's about how best to start the bike. The golden rule is don't roll the throttle when kicking. If you do it will never start.
When you look at the bike take a look in the airbox - the condition of the box and the filter will tell you whether or not regular maintenance has been carried out.
Lay the bike on its right hand side and take off the timing cover (on the left and assuming the owner is OK with this of course). You will be able to feel if there is any movement in the crank bearing or not. If you cannot do this, drain the oil and see how clean it is and if there is any metal in it (you can simply refill the bike again once this is done so no harm done if you don't take it).
Check the radiator fluid level and if the fluid is clean (not milky which indicates a potential head gasket problem). You can also check this by removing the small screw on the right hand crankcase down by the oil filter
Another check is to lay the bike over on the left hand side and remove the oil filter cover - check the colour of the oil and the cleanliness of the filter - again an indication of how regular the maintenance has been.
Generally however, you will just have to go by gut feel, the general condition of the bike, and how genuine you believe the owner is.