PC with possible smoke damage,but it booted. What are some test I should run on the MB, PSU or Ram etc?

Smoke leaves soot, soot is carbon, carbon is electrically conductive.

It may be there are soot deposits on various components causing false pathways. Take it all apart and use a 1" paint brush, or hoover with brush attachment to clean the circuit boards of every component, including the cpu and cpu socket, PCIE slots etc.

The act of removing the HDDs may have been enough to disturb soot deposits and break any circuits the soot was causing, but still go over their circuit boards and clean them..

Other concerns are soot INSIDE the psu causing issues; if you are comfortable taking it out of its casing, remember to give it 30 minutes to fully discharge or you might get a zap from the capacitors.

Run the Intel Processor Diagnostic Tool to test the CPU followed by Memtest86 (bootable USB thumb-drive) for RAM. In many ways, both utilities will provide some overlapping tests. So if both pass the test, I'd say you're in very good shape all things considering.

As for the drives, be sure to backup your data ASAP. Honestly, no telling what condition they're in if they've been exposed to an excessive amount of heat. But if you want to test that WD Digital Blue drive, look for "Data Lifeguard Diagnostic for Windows".

I'd just replace the fans and move on, if there are/were memory issues, you ill get freezes/reboots/hangups....

I've had pretty good luck with a can of CRC QD Electronic Cleaner. You can order it on Amazon or purchase at a local automotive store such as O'Reilly Auto Parts and AutoZone. It will clean the contacts. But, I'm not sure if a (soft) paint brush is required. The most worrisome about it is applying too much pressure or force on the CPU socket, a single bent pin and it's all over with! So extra caution is needed. Oh, and use outdoors. You don't want to be breathing this stuff in while indoors. FYI, It's best to leave the parts that's been sprayed outdoors for at least 10 minutes when done as I'm sure anyplace that might be wet will still out-gas while drying.

http://www.crcindustries.com/products/qd-174-electronic-cleaner-11-wt-oz-05103.html

On the more extreme side of things, others have actually washed their Motherboard, CPU, and RAM, and other components in the dishwasher (remove CMOS battery first). For sure, this will clean all sorts of residue, but that's a bit dicey IMHO. If you live in an area where you have lots of hard-water, I'd suggest pulling the parts out after the wash cycle and rising them off with a few sprays of isopropyl alcohol. Last thing you want is for hard encrusted mineralization deposits (calcium and lime) to form on the contact points as it dries out in the heated drying cycle. Don't trust the detergent in this regard to rid you of this for you as it's not always effective.

Just to be perfectly clear, I've used electronic cleaner, but never the dish washer method. But I've heard of great success using it. Again, do your research further before proceeding. A few things I would be cautious though is not to clean any moving parts such as the fans as it will strip the bearing clean of its much needed grease. This will ruin the fan and keep it from spinning freely. Also be mindful of any attached heat-sink accessories on the motherboard. After cleaning, you might have to detach them and reapply thermal grease. If they're held in place with thermal double-sided tape, and it becomes detached, then you'll need to reapply with a fresh strip to allow proper adhesion again. Oh and if you really want to wash that PSU, be sure to remove the fan, and wait at least 24 hours with the unit unplugged as the capacitors may still be charged with flea power before washing. Also, wait at least 24 hours or several days before reassembling and using. If there's any water left in the PSU, you'll either damage it, the motherboard, GPU, or cause great harm to yourself...or worse! Honestly, for smoke damage, I'd say just replace the PSU outright and not even attempt cleaning it. I'm not going to give you advise I wouldn't be following myself.

That all said, good luck.