Use the counter which shows how many characters you have used so when you are just under 160 characters send your text then continue with another text.
The 160 limit is the industry standard across all networks as this is the format all handsets use, so it is down to you how ensure that you do not send a text longer than 160 characters because you will be charged how ever much you argue as it is in the terms and conditions.
If you would like to read up on how SMS come about click
HERE