Now we came to discuss a subject some traditional keris expert reluctant to discuss. It's the isoteric subject, or the "X" factor.
What makes a keris "pusaka" while others are not ? Nechesh had made a good point, one of them is Family Heritage / Heirloom. But heritage isn't the only way for someone to own a pusaka-rated keris. Sultan Hamengkubuwono V (reign early 19th C.) in Jogjakarta known for buying some kerises and made them as Court Heirlooms. The "Kyai Pamungkas", once a keris pusaka belongs to Prince Tejokusuma, were acquired by buying it for 1000 gulden (early 20 c.). Sure, family heritage heirloom might had a long story, but in fact, every keris had it's own story, isn't it ? So, what the "pusaka" really is ? IMHO, pusaka might be interpreted as "Masterpiece". In keris's world, it MUST fulfill BOTH the exoteric and isoteric criterias. It were the Empus who really made some kerises rated as "pusaka" or "ageman", and only Master Empus who able to made such (considered) "powerful" pusaka. (In fact,most of the court in Java consider a spear/lance as their most powerful pusaka, not the keris). According to Java's tradition, pusakas were named Kyai (male)/Nyai (female) ..<something>. Usually, Kyai/Nyai is a title given to respected elder or spiritual teacher. The name isn't only a form of admiration to the art, but in fact, the pusakas itselves are really "teaching" a lesson and the empu's intention to the owner/spectators, in symbolic languages. Only those who understand the language might learn the precious lesson the pusaka (ultimately, the Empus) tried to convey. Treating pusaka as only an amulet is really a derogating way, if not considered as humiliation to the Empu. But treating it as an ordinary blade with no respect at all might also do so.
Thus, in old days, looking at someone's pusaka might reveal his philosophical view and intentions, knowledges, and also his identity. For many Javaneses, it might felt like naked. So, they try to "hide" this, by very selectively showing their pusaka, and also, by using the "ganja wulung". But today, it might means nothing, because there is so little keris owner who understand the language, and I'm afraid, many of this symbolic languages had lost.
But in the end, it is the owner who makes the decision, whether his keris is pusaka or not. What ever the owner decides, any wise spectator should treat it as a pusaka.
So, how could we tell ones is pusaka while other isn't ? As I mentioned before, a pusaka must fulfill both exoteric and esoteric criterias. Exoteric very much related to quality, those are, the materials used and the workmanship, which certainly shows the empu's mastery. The esoteric is much more difficult, as Alam Shah already said, just use your gut feeling
Sorry, seems I give no help at all. But for the hints : pay attention to the iron. Old book say "the steel gives the sharpness, the iron gives
the power, the pamor gives the glow/shine". Contrary to popular belief, pamor should be considered only as a "book cover/title", where the empu put his title to his intention. It is the iron, which really contains "the power". No named pusaka (that is, pusaka which bear a name, "Kyai") I've already seen had bad, rough and porous iron. In fact, many of them just show a minimalistic pamor, scattered-rice type (beras wutah), which convey a high-degree philosophical lesson. Good understanding on keris making process and metallurgy will show why.
Last but not least, I must admit that what I've been saying is only from Jogjakarta's view. Surakarta might say something differently, because of the different philosophical approach. Bali, Makasar/Bugis and Malay will certainly say something else.