If you have the means and are serious about improving then I highly recommend buying a power meter. I have Favero Assioma pedals which are about the cheapest option for dual sided power and work well. After I've been riding for a while I tend to slack off and fool myself into thinking that I'm working harder than I really am. The power meter keeps me honest.

But without a power meter you can certainly pace yourself by heart rate. Figure out your heart rate zones and then do some test rides holding your heart rate steady in a particular zone and sustain the effort as long as you can. Then you can estimate a maximum target heart rate for each climb segment based on how much time it should take.

You can also experiment with different pedaling cadences. Every cyclist has a different optimal cadence, but in general you want to shift down when climbing and keep the cadence high (unless you're intentionally doing a low cadence strength workout).