You can check a specific URL segment. For example if you need to load some CSS on the About page and that URL is yoursite.com/about you can do this:
{% if craft.app.request.segments|first == 'about' %}
{# Load CSS #}
{% endif %}
You could also check by entry.id
{% if entry is defined and entry.id == '123' %}
{# Load CSS #}
{% endif %}
Replace "123" with the id
of the entry you'd like to load the CSS for.