Things to look at, in no particular order, would include:
blade shape - is the blade straight or does it have a bend?
kizu -are there any flaws and or chips in the blade,
hada - consistency, tightness, delaminations,
hamon - consistency, hataraki,
nakago - patina, file marks, mei, ana (number type),
habaki - fit, quality of materials used e.g. gold vs copper
shirasaya - fit, condition, stains, dents
tsunagi - does it fit both the shirasaya and the koshirae.? Is is damaged in any way, if there is a wood habaki - has that been damaged.
koshirae - quality of individual pieces, signatures - gimei?
theme of pieces - do they work well together,
fit of pieces to the blade - tsuka, fuchi kashira correct size? seppa, tsuba, kozuka, kogai do they work well with the tsuba?,
Does the koshirae add to the blade or are they poor quality or so good that they are a distraction.
Finally, how does the whole make you feel. I have picked up some swords that made my hands sweat, that made me feel awe, that made a connection to me in some personal way. I do believe that the sword finds the man/woman. In some cases, the feeling was almost immediate.
My mentor once told me: "You only regret the one you didn't buy."
If you have done your due diligence, and the blade "speaks to you" make the purchase happen.