how to know mysqli extension is installed / activated or not?

You can run the following command via root.

Code:

php -m | grep -i mysql

You will get an output like below.

Code:

[root@yourserver ~]# php -m | grep -i mysql
mysql
mysqli
pdo_mysql

If you do not have root access you can create a php file with the following code.

Code:

<?php
phpinfo();
?>  

Then just search the page and MySQLI should be there if it's installed.