Some basic points to tell if its been restored or not:
Turn the engine over and look on the inside of the cab. See if the color on the inside matches the outside. If not, its been restored. Same with the passenger cars.
Many original pieces show a drip line (a dried blob) which resulted because they were dipped and then hung to dry so paint would run off and sometimes dry in place before it would drip off. Restorations typically won't have any. Be careful with this one, because many originals don't have a drip line, but when you see one, then you can start to be more confident that its original paint.
Ask the current owner about the history of these pieces. If he doesn't know, ask if he purchased them as a set or individually. This second question won't tell you if its a restoration or not, but if they were purchased as a set then I typically focus on one piece and if that appears to be original, I have more confidence they all are.
Look at the lettering on the engine and on the passengers cars. When you hold them up to the light, you should not see anything in between or along the edges of the letters. Originals were rubber stamped. Many restorations use decals, which typically leave a transparent halo around each letter or you can see the transparent portion of the decal for the entire word if the restorer didn't cut out and mount each letter individually.
If there are any scratches, see if there is a grey primer visible beneath the paint. Originals had no primer, so it should show bare metal. If there is primer visible, then its a restoration.
Ask the current owner to take off the roof of the passenger car. Look inside and find the tabs that hold the parts together. See if there are any missing or if there are knicks or scratches by them. If so, then someone has taken it apart for some reason and its likely a restoration of some sort.
Check the latch couplers. When you open them, they should gently snap back down. If not they are replacements.
There are other things like cast headlights versus strap headlights, but I have to get out a reference that I don't have handy for the 248s. Even with all the above, the 248s are great little engines (I have 3) and the 629 four wheel passenger cars look great being pulled by it. Good luck!