Identify SSTI
The most effective way is to inject special characters with semantic meaning in template engines and observe the web application’s behavior.
${{<%[%'"}}%\.
Since the above test string should almost certainly violate the template syntax, it should result in an error if the web application is vulnerable to SSTI.
Identify Template Engine
There are SSTI charts out there that usually starts with injecting ${7*7}.
Follow the flow chart and try out different payloads until you find out the template engine.