Fever?

My bfs niece and SIL are sick .. Idk if I'm being paranoid because she hasn't had her shots yet but today she feels a little warm to me and she's being very very fussy. Way more than usual. She just wants to be held and won't stay asleep .. I have a thermometer how do I properly check her temp? Or should I just take her to the clinic tomorrow as a walk in ? Her next appointment isn't until the 15th. My poor baby I hope she's not sick