There used to be a handy site http://sni.velox.ch that'd quickly tell you if your browser or a proxy along the way was preventing you sending an SNI header.
I have an IoT device which I can cause to visit any site (and see text output from) - and I just want to see if it is correctly sending SNI. Because it is GSM, it's not possible to break out tcpdump locally.
https://cc.dcsec.uni-hannover.de/ is a test for ciphers and SNI.
If you can get that page on the device and you see under "SNI information", "Your browser did not send SNI information", then whatever https libraries you're using don't support SNI.