I can recommend Annie's method of measuring max (although, as said, take someone with you)

When I first started riding I was in the position of having a 10-11% short stretch to climb up about 1km before my house...

I can get up there alright now, but there is a longer hill I can do this on, but riding up there was almost the "ride til you puke" sensation... and when you throw up you know you are maxed out...

I have never vomited but once you approach that level, you know that your heart rate will be up there.

*warning
do not try this alone