The short answer
Before spending on ads, make sure form submissions, phone calls and purchases are tracked as conversions, that each is tied to its source, and that you have tested them yourself. Without this, the ad platforms optimise for clicks and you cannot tell which campaign paid for itself.
Form submissions
Track the successful submission, not the visit to the contact page. Send a test and confirm it appears.
Phone calls
Use call tracking or at least click-to-call events. For many local businesses, calls are most of the leads.
Purchases with value
Stores should pass the order value so return on ad spend can be calculated.
Source for every lead
UTM tags on ads and a hidden field or CRM note so each enquiry records where it came from.
Platform conversions
Import or set up the same conversions inside Google Ads and Meta so bidding uses them.
Consent and privacy
Check what consent you need where your customers are, and that tags respect it.
A test from start to finish
Click your own ad, submit the form, and follow the lead through to wherever sales sees it.
The takeaway
This is a few days of work that decides whether months of ad spend can be judged. It is the first thing a careful agency does.
Common questions
Do I need Google Tag Manager?
It is not required, but it makes adding and changing tags much easier and keeps them in one place.
What if I already ran ads without tracking?
Set tracking up now and treat the next month as your baseline. Past spend cannot be attributed after the fact.