Is there any way to guage accurately the potential COV of a resale HDB?

Since the COV depends on the HDB home valuation of a flat, you can actually check that first. We offer a free home valuation report, complete with market trends and past transaction data of units in the same area. https://ohmyhome.com/en-sg/home-e-valuation/

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But there's really no way of confirming the COV of a property until you've made a deal with the seller.

Can try using valuation tools on SRV or 99 to get a rough estimate. I was told by the seller agent (I bought a resale a year ago) that in the current market, buyers should be expected to fork out COV for resale. Not sure if that is still the case now

You should consider using 99.co's Property Valuation Tool. Its free and helps you get an estimate for the market value of the HDB you are looking at.
Depending on the actual selling price, you can use the tool to get a rough indication of how much you might need to fork out! https://www.99.co/singapore/property-value-tool

Found this website to be relatively accurate. Usually your agent should have a good gauge/sense of the valuation as well.

https://www.srx.com.sg/mysg-home