Depends on the firearm and your taste. If for instance an AR platform or 1911- The sky is the limit. The amount of mods you can make to either is almost endless.
The big question is why do you want to modify it?
Does the modification have a practical purpose such as a beavertail safety on a 1911 or do you just "Want it to look tacticool"?
You would be amazed at the amount of people that will spend beacoup dollars to make their firearm "Look Cool" only to add no real function.
Perfect example-We get guys in the shop almost daily that want quad rails on their AR and then proceed to hang every piece of crap they can find off of them.
What have they accomplished is taken a perfectly functional 6 pound firearm and turned it into a 12 pound boat anchor!
I can gaurantee none of these guys will ever "Step through a doorway" in a home sweep with their rifle. If they hear a bump in the night they will cower in the corner while reaching for the phone.
If the modification has a practical, REALISTIC use then by all means, make your firearm into the tool you NEED for the task.
Otherwise why waste the money?
As far as being able to tell if the modification was done, that is the sign of a proffesional. If the firearm looks and functions like it came from the factory with the mods on it, then the job was done well and you should be ok.
It is easy to spot amateur work. Punch marks, scratches, things dont line up right etc.
Usually if it looks crappy, it is.
Some things a person can do easily though. Such as a "drop in" trigger group for a rifle etc.
If in doubt-take it to your local 'Smith and have them look it over.