It's in the little things, which add up over time, that let him know.
And he isn't sure if he really even realizes it, sits downs and defines this for what it is and all that it implies, or if his brain just started keeping track one day and running comparisons, and never really informed him what it had found.
It is the way he tilts his head to the side sometimes. Or a certain phrase that he might say. A way of styling his hair, or the look in his eyes. The blush that he gets, on nearly a daily basis. His smile. His rare laughter. His seriousness, certainly, but then also his franticness over seemingly the strangest things. How he interacts with others, once in a while, causes a click, and a memory to spark. Mostly, how he is with Italy himself, and then he can't forget a younger blonde boy in stiffer clothes, who had promised to come back home....
He doesn't voice this, not even to the people who had shared a house with the two of them all those centuries ago. He certainly doesn't say anything to Germany, worried that somehow he is missing all of those memories for good reason, some trauma too terrible to recall blocking all the better moments which they had shared.
But it causes him to cling to other perhaps a little tighter, to regret the moments in which he is obnoxious a little more, to seek to be the best friend – and sometimes more-than-friend – that he can be. He desperately tries to make up for every imagined or real indiscretion that he might have committed against the other, tries with all his being to make days easier and more relaxed for his former – and maybe current – love, works hard to please and to do as told.
He can't imagine another century – another decade, another year, another day, even – without the man of his childhood and of his youth right there with him. He works at swallowing down his fears then, and relishing what they have now, all while praying that there may never be another event that calls the other away from him once again. He isn't sure that he could handle a night without the other safe in their bed, with him.
He had already made that mistake once, and once was more than enough.