Look to the light! I'm getting better at identifying the paper, but it really helps to have two examples to compare side by side. Held up to a light, for some numbers, the hard paper practically glows and is translucent whereas the soft paper is much more opaque. Here's a 150 and a 188 held up to a light, looking at the back. In addition to the opacity, you can see a gridlike pattern in the soft paper.

The color can be a tell as well, I bought the 188 which was described as a 161. I could tell from the posting's picture that it was very yellow and figured it was the 188.
The 5c Taylor is a tough one, at least with the two examples I have:

Some days I call the one on the left a 179 (like today) and other days I swear I can see that soft paper grid in the lower left. The grid on the right is very plainly evident, and so certainly a 185.