Ling Lee, that is a really hard question to answer with no context!  But I'll give it a go.

The very first thing you have to do is decide whether you are going to be upfront about the fact that you are a (potential) competitor or whether you are simply going to pose as someone interested in the area and see what they will share.

There are cases where similar businesses are not competitors - often because they are in different locations and customers want someone local, or possibly two restaurants on the same street offering different cuisines - but more likely you will find it is not in their interest to share with a potential competitor.  So an honest approach may limit the amount of information you get.

If you want to be honest (and depending what stage your entrepreneurial friend is at), one approach is to explain that you are trying to decide between a number of niches and you'd really appreciate any information about whether this is a good niche to invest time and money into.

If you're happy not to disclose who you are, you can often get a lot of information by ringing up posing as a potential customer and just asking questions chattily to see what comes out of it.

Whatever you decide, the general rules for gathering information are a) be polite and friendly b) respect their time and if they prefer fix a time to call back which is more convenient for them c) say thank you and d) ask them what other sources (publications, associations, websites, individuals) they think might be helpful for you in your research.