As the title says, how do you know if your slave is pregnant? I mean where can you get the info

There's a cheat to detect pregnancies at a very early stage. Visit the slave guild, click "Services" and select your slave:
- if the "Sterilize" service is available, she is not pregnant
- if the "Commit Abortion" service is available, she is pregnant and she has completed at least 1/6 of her gestation period
- if neither of these two services is available (and your slave has a working womb), she has been impregnated a few days earlier (less than 1/6 of her gestation period)

If you appoint someone as Headgirl, she will alert you if a slave is pregnant, but not immediately. In the daily report you will read:
- at 40% of her gestation period: X appears to be pregnant
- at ~76% of her gestation period: X will likely give birth soon

If you open the slave's profile, you may read:
- at 25% (or more) of her gestation period: Her unborn fetus causes her belly to bulge slightly
- at 50% (or more) of her gestation period: Her advanced pregnancy is clearly evident by the prominent bulge in her belly
- at 67% (or more) of her gestation period: Her unborn child forces her belly to protrude massively; she will give birth soon

best option is mind read from mage guild, i go for this and green potion to remove unwanted pregnacy

After intimacy just open Statistics, there is count how many times she got pregnancy. New slave has 0 count. 1 mean that she is preg.

As AK says, I also use the statistics view of a slave.  I compare number of times pregnant versus number of times given birth.  If the 2 numbers are not the same, she is pregnant (although this method stops being quite accurate if a slave ever aborts/miscarries).