Speaking from experience, owning 2 DK cabs:
1) Test EVERYTHING before buying - controls, buttons, coin mechs, start a single player and 2P game and run through some paces in 1P and 2P, marquee light, of course monitor and sound. Check that the coin box is present. Look at pictures of the inside of cabs, so you know what it should look like. I will gladly post pics of the inside of my cabs, if you would like, just PM me.
2) Look at the metal tag above the rear door. It should have a TKG part number on it (TKG2, TKG3 or TKG4). If it does not, it is a conversion. If it is missing, it could be a conversion. Maybe that's a non-issue for you, as long as it looks the part
3) Inspect the entire exterior and assess the condition.
4) Original paint is glossy, very smooth finish. Not to say a re-paint couldn't replicate this, but if the finish isn't, it's definitely a re-paint.
5) Open the back. Know that there are 2 types of power supplies that could come in a DK - PP-7A and PP-7B. The part number sticker is on the side of the PSU facing toward the coin mechs. You can take a picture of that side of the PSU by reaching into the cab, and should be able to clearly see the part number sticker.
6) Remove the control panel and check the wiring - unlock two latches on either side of the control panel by reaching through the coin door, then lift up on the front of the panel and gently move the panel forward to free it from under the bottom bezel retainer. you can then gently flip it over to inspect the bottom of the panel and the wiring. Note, original Nintendo CP's have an unused button hole that you don't see through the CPO.
7) Know that there are 2 types of iso's that could come in a DK - black (PT-8a) and gold (PT-821b)
Remove the bezel and inspect the monitor. You can't always see screen burn and scratches on the monitor through the smokey tint on the bezel
9) Inspect the marquee for small cracks. These can also be difficult to see unless you are looking closely.
10) I suggest reading this thread on KLOV, detailing many owners' DK cabs including the original converted radar scope cabs (TKG2) and TKG3 red cabs (before nintendo officially went to blue for the remainder of TKG3 and the entire run of TKG4's): https://forums.arcade-museum.com/showthread.php?t=64571
. It's a long read but will give you intimate knowledge of all the nuances to look for when looking at DK cabs.
11) Determine whether the cab has a 2-board stack or 4-board stack. Since you are presumably getting the cab with the intent of submitting score runs, you will be safe if it is a 2-board set. If it is a 4-board set, then you want to inspect the title screen to ensure it says "(C) 1981 Nintendo of America Inc.", you are good. If it says only "(C) Nintendo 1981", then the code contains the ladder trick, and is not acceptable for score submissions. Board sets with original code are actually more collectible, and you may want to buy it as is if the price is right, but will need to swap some of the ROMs to make the board set acceptable for score submissions.
12) Watch the monitor through a couple cycles of the attract mode, let it run while you are checking out the rest of the cabinet, then watch it again. Ideally, you will let at least 15 minutes pass from first watch to second watch, as that's how long it takes the video components to fully warm up, and any issues to present themselves. Look at the colors. Are they bright and crisp? If not, you could be in for a cap kit at a minimum. Worse case scenario, the tube could need to be rejuvenated at a minimum...worser case scenario, the tube needs to be replaced. Take it from someone who recently replaced one of his DK monitors - good Sanyo 20EZ replacements are hard to come by. If there is one thing to focus on making sure it is good in the cab, it would be the monitor.
:edit: 13) Look at the power cord. It should have 3 prongs on the plug. Many have the ground prong broken off. It will still work, but it is always best to have a piece of equipment grounded, which is intended to be grounded.
14) Inspect the instruction sticker and the side art. Are there signs of replaced side art, under the monitor carriage bolts? Know that early red DK's (TKG2's) did not come with side art. Everything TKG3 and TKG4 should have side art. Just more things to inform your decision and speak to the overall condition of the cab.
If you happen to find a red TKG2 cab with a TKG2 board set in for a good price, and it is in good condition and fully working, buy it. Highly collectible, who cares if the board set is acceptable for score submissions. :p
Lots of info, I know, but hopefully this helps. Of course all these things are subjective, and it is up to you what things you can fix if needed, live with if not perfect, etc etc. Once you have gathered all the information on a cab you are looking at, then decide if it's in good enough shape to be worth your while, or possibly it has too many issues and move on to the next one.