Two step process:
1) Pour yourself a sample of standard commercial American ale. Swoosh it around in your mouth. With your tongue, feel around inside your mouth, especially the roof of your mouth. Okay... that's "normal". That's the baseline.
2) Pour a sample of your lager. Swoosh it around in your mouth. With your tongue, feel around inside your mouth, especially the roof of your mouth. Does it feel a lot more slick or slippery in there this time? That's diacetyl. But if you go back and forth and you can't detect it in the mouthfeel, then you *might* not have diacetyl. You might still need a judge to know for certain.
Hope this helps.