How to grade / judge / evaluate a steganography approach?

When it comes to evaluating a text steganographic approach, I think we have to consider these things:

1. How unlikely it is for someone to guess that there is a hidden message in the message being sent?


2. How large a hidden text can be stored in a cover text of fixed size (larger the better)?


3. How hard is it for someone to decode a message should it get suspected?

It is easy to quantify 2 and 3.

But how do I arrive at a quantitative way of measuring 1?

PS: Please assume it is a text-in-text steganography approach.

Are there any metrics for that?