A 15 Amp recep. has a hole for the ground, a shorter slot for the hot side and a longer slot for the neutral.
A 20 Amp recep. has a hole for the ground, a shorter slot for the hot and the neutral slot looks like a "T" lying on its side.
What is the approximate distance from the humidifier and the outlet?
How long is your extension cord? I just reread....25 feet.
You should be fine with what you have. I just tend to use the shortest cord possible. I'm staring at a 15 foot cord from ACE Hardware that is 16 Gauge, 13 Amp. A humidifier will have a larger pull on the line when the compressor comes on and will run at a lower amperage when up to speed. I really doubt it would ever reach 10 Amps peak even on start up. (Think of a refrigerator or window air conditioner.)