How to query or measure packet latency between a Elastic Load Balancer(ELB) and a EC2 instance behind it?

My supervisor asked this question but I'm not sure how to scientifically measure it, currently I make several requests to a HTTP service and measure the time_starttransfer cURL statistic with the domain name resolving to the IP address of the ELB and the EC2 instance and subtract the numbers to provide an answer but I'm not sure this is the proper way to do so.

I also checked the CloudWatch dashboard and unable to found similar metrics, is there any?

I would like answers for either the NLB(OSI layer 4) and ALB(OSI layer 7), thanks in advance!