@Katlynn Teague The short answer is that there are a lot of things we will not know since we aren't doing a home inspection. And I want to note that the reason we don't do one is so we can approach a seller and make them an offer that is 100% rock solid with a non-refundable EMD. We don't do it arbitrarily just to take on more risk haha.
In exchange for this we can generally get the property at a lower price than if we had contingencies and hoops to jump through. We still walk the properties and take lots of pictures, so it isn't like we are doing this site unseen. We just forego the home inspection to make our offer more competitive.
Back to how we find out the things needing fixed.....we will have a "fudge factor" in our analysis where we budget for surprises. HVAC is pretty easy....if it looks 15-20 years or older, it likely needs replacing. Even if it works properly, a first time buyer we are selling the house to once flipped will be freaked out when their inspector notes the HVAC is old. So, likely makes sense to just budget for replacing it anyways. The roof is visual too and you can tell if a roof needs replaced or not.
Plumbing and electrical are hidden (unless you tear open the walls) but even if both need fully replaced, which is highly unlikely, you're only looking at $10k or so depending on the size of the house. For our first flip we had to redo all the wiring and most of the plumbing and I believe it was well below $10k.
So even if all surprises amount to $10-15k, the deal should be able to absorb this in my opinion, especially if you have a bit budgeted for it. If the deal can't support it then the deal is probably too thin to begin with.
The only exception here is well/septic. We still will usually get inspections done on these because that is something we deal with less frequently, and can be a much larger expense.
When you walk a property, focus on the obvious stuff like age of roof, water heater, HVAC, and everything else you can see, and then just ask good questions of the seller to try to find out if there is anything underlying. We've never really seen people intentionally covering things up, it is actually the opposite where most sellers give way more info than we needed. Hope this helps!