Speech signal formant spectrogram. How can we determine F1? The formant spectrogram in this page points F1 and F2 as below;



In bottom-right F1 movement, I thought F1 could be below blue one. But how can we determine F1 is upper one not below one? 



Is it because we know F1 and F2 have to be getting closer from [ε] to [ae]? Or, can we determine F1 only with spectrogram?
You should be able to see formants very clearly as peaks on the spectrogram slices. The 1st, 2nd ... are the several peaks. In the picture above (in the blue area) the first two peaks seem to have very similar power and so this can introduce errors in the estimation algorithm.

You can use Praat and "Show formants" option to track formants and observe their behaviour over time. You can try to make continuous transitions from vowel to vowel and then try the same vowels with a silence in the middle and observe the differences.

If you are developing an algorithm you can use a Kalman filter for tracking formants. (maybe this helps: http://link.springer.com/content/pdf/10.1007/978-3-540-85980-2.pdf#page=257)