sounds like a daft question but what/where is the simplest and easiest place to measure to be able to tell? im sure you can tell with time just looking but i need to know for sure…

Does your head tube look like its about the same size as a coke can?

If yes, 1.5

If No, 1 1/8

Or far more simple, take it to your LBS and get them to tell you?

Or go to the website for your bike and check out the specs?

Take the top cap off & measure it

I was doubtful a vernier calliper is in the tool box, but yes, that too

I was doubtful a vernier calliper is in the tool box

just use a rule

What kind of bike is it and how old?

Can you take a look at a bike that has an 1 and an eighth headset and compare yours with it. one inch will look a lot skinner in comparison. Or take the stem off and measure the steerer tube diameter.

top cap is good idea but what of it doesnt have an ahead stem?

Measure the top of the quill stem with a ruler.