Is there a trick to find out the intensity of words with similar meanings?
Who told you there is an intensity difference there? 

你觉得它们不一样，不过我不这么认为。
你觉得它们不一样，可是我不这么认为。
你觉得它们不一样，但是我不这么认为。
你觉得它们不一样，然而我不这么认为。
你觉得它们不一样，而我却不这么认为。


They sound absolutely the same to my ears. You use different words only because you want it to sound slightly more colorful. 



Update: I might be wrong. See discussions in the comments. 

Anyway it is easy to remember:


不 in 不过: 4 strokes
可 in 可是: 5 strokes
但 in 但是: 7 strokes


See? The intensity goes up with the strokes.

却 is a different beast and shouldn't be compared with these three.