Verify hours on KTM 450SX-F?

What year is it? The newer ones come stock with an hour meter.

Yeah I think the KTM dealer can check the ecu. But if they bike had an Aftermarket ecu the numbers will be off.

The ECU will tell you everything. It lets you know how much part- or full throttle the bike has seen and the day and hour when the bike was ridden. So if there is a long gap of several weeks or months between riding time you could be quite sure that a different ECU was used. Afaik from model 2016 upwards

Not always true. If there is a gap it could be from sitting.

Also, any ktm dealer can read, but also reset the ecu hours, and the evu only records in 6 min intervals, so if fully run the bike for 5 minutes, then shut it off, it won't record any time in that scenario.

Exactly. I was hurt for 3 months and only ran my bike for 15 minutes the whole time. If the OP doesn't want to trust the hour meter on the bike he needs to buy a new bike.

What year is it? 17+ come from the factory with a hardwired hour meter.

Earlier models, yeah you have to read the ECU

Check bolt heads, especially on the engine. If never been opened they will have only slight marks from tightening in the factory. Check the footrests and rear brake pedal for wear marks. If allowed, pull the front fork legs out and see the amount of discoloration/abrasion on the aluminium tubes where they sit in the yokes - this will give an indication how much and how hard the bike has worked. Check the frame coating behind the leg covers - if coating/paint is worn away - the bike ran in sandy/dusty conditions for quite a while. The upper shock bearing will develop a little bit of play way before the other bearings - but not within the first 10 to 20 hours. Look at the front and rear wheel spindles and nuts, they will tell you how often they were touched. Look at the oil drain/oil filter screws. Check the brake discs and the pads for wear. Look at the screws holding seat and tank spoilers. They only need to come off when valves are adjusted or the frame under the tank is to be cleaned, or an electrical problem occurs - or the ECU has to be worked on ;-).
On all my KTMs the rear upper shock body is slightly rubbing on the inner side of the air hose between filter and injector, the amount/absence of wear on the hose is a solid indicator of the hours this bike was ridden.
The front and rear chain guide will tell as well how many hours they were in use.
Generally - if you like what you see and the vendor seems trustworthy and not telling a lot of shit, allows you enough time to inspect the bike thoroughly and your gut feeling is "yeah, I want it!" - then go for it. The bikes are really good, I did 165 hours on my 16 SXF 450 without any engine problem.