I needed to check the certificate expiry date in an NUnit test. The below snippet can be used to check any certificate properties, using a custom certificate validation callback. All you need to do is read the properties you are interested in within the callback so you can check them afterwards.

DateTime notAfter = DateTime.UtcNow;

var httpClientHandler = new HttpClientHandler
{
    ServerCertificateCustomValidationCallback = (request, cert, chain, policyErrors) =>
    {
        notAfter = cert.NotAfter;
        return true;
    }
};

using HttpClient httpClient = new HttpClient(httpClientHandler);
await httpClient.SendAsync(new HttpRequestMessage(HttpMethod.Head, url));
            
Assert.IsTrue(notAfter > DateTime.UtcNow.AddDays(60));

This code only depends on:

using NUnit.Framework;
using System;
using System.Net.Http;
using System.Threading.Tasks;