For a man,
it shouldn't really matter whether she's being polite
or whether she's genuinely interested.

One of the important differences between men and women
is that men tend to warm up much more quickly to a romantic situation than women do.

On average, women just take more time:

For the interest to develop.
For them to realize that they're looking for something,
that they're interested in someone romantically.
Men will tend to know this a lot faster.
Just on average, in general terms:

Men heat up quickly, then they also tend to cool down quickly.
Women tend to warm up more slowly, but then they also retain it longer.
The author, Doctor John Gray, talked about how:

Men are like blowtorches, and women are like ovens.
The blowtorch shoots up real fast, it burns bright,
but then it can also turn off real fast.
The oven takes a long time to get the heat going up,
but once you've got it up there, the heat actually retains itself quite a bit,
and it takes quite a while to cool down.
So back to the question of:
"Is she just being nice or is she actually interested?"

She might not know right away.

And as a man, it's important for you to:

Know what you want.
Not push it, not put a burden on her with what you want.
Make it nice and clear with what you want.
Keep moving forward with it.
If she's being polite and she seems to be warming up to it,
continue going forward.

You'll know in yourself when it feels like you're starting to maybe waste some of your energy.
You'll feel something inside, and that's the time to become more explicit about it and be straightforward and ask.