Moira in N.E. England

I’m experimenting with cooking joints of meat in my slow cooker and thought it would be useful to use a meat thermometer.

I purchased one in my local supermarket and have never used one before.

My problem is the instructions do not tell you when to insert it. Should it be in from the start of cooking or do I just use it when I think the joint is cooked?

Thanks for any help you can give me.