1. He goes out of his way to spend time with his family that day. As demo'd by the grandma example. While sweet, turns out there may be some ulterior motives lurking. Whomp whomp.
2. He crutches on the "work" excuse more than a week out. "If he's building up talking about a big proposal he's putting together and it's just consuming him, [saying] and he'll be no fun this week or something like that" it's bad news, says the resident Valentine's Day-avoider. How terribly cunning.
3. He totally overplays it. If he's really a jerk, he could talk about so many grand gestures that he's going for the reverse psychology approach--he'll tire you out to the point where you say, "You know what? Let's stay in."
4. He hasn't even laughed about it. Another friend of mine said, "A lot of us will joke about it and downplay what we're going to do." They'll at least acknowledge that the Valentine's Day exists, even if they're not a big fan. Don't take his silence as a sign that he's just being hush-hush, and that he's got something up his sleeve, though; if he's not using humor, he's likely playing an avoidance card.
5. He says he gets depressed around this time of year. It may be true, but if his depression only lasts from February 12 through 15, start making the squinty-eye at him.