Will I be able to tell if I am having twins during in the 1st trimester of pregnancy?

Yes. An ultrasound performed in the first trimester can confirm twins. Sometimes when twins are diagnosed very early, one of the twins may not survive and then the pregnancy usually will proceed normally as a singleton. Blood levels of the pregnancy hormone (HCG) are also higher than a singleton pregnancy.
An ultrasound. At 6-7 weeks can tell, no need for u to tell.