If you have a temperature and humidity sensor, you can calculate the dewpoint using the Magnus formula. See for example the german Wikipedia.
It is pretty straight forward to create a template sensor out of this:
template
- sensor:
- name: "Dew Point"
unique_id: "indoor_dewpoint"
state: |-
{%- set t = states('sensor.indoor_temperature') | float %}
{%- set lnrh = log(0.01 * states('sensor.indoor_humidity') | float) %}
{%- set k2 = 17.62 %}
{%- set k3 = 243.12 %}
{%- set q1 = (k2 * t) / (k3 + t) %}
{%- set q2 = (k2 * k3) / (k3 + t) %}
{{ (k3 * ((q1 + lnrh) / (q2 - lnrh))) | round(1) }}
unit_of_measurement: "°C"
icon: "mdi:thermometer-water"
device_class: "temperature"
state_class: "measurement"