If it's a gateway to a paid service (music stores, Uber, Lyft, etc), it generally won't have ads. If it's free, but not a gateway to a paid service, I check to

If it's a gateway to a paid service (music stores, Uber, Lyft, etc), it generally won't have ads. If it's free, but not a gateway to a paid service, I check to see if source is available; if source is available, it generally doesn't have ads (and if it does, I can just build it myself and remove them). If the above checks fail, I check for a paid, ad-free version. If such a version exists, that implies the free version has ads. If the price is reasonable, I'll buy it, otherwise I'll skip it. Finally you have free-as-in-beer but not as in freedom apps, that don't have paid alternative or any other way of generating revenue. I just assume those apps are either encrusted in ads or scams, so I don't install them.