warframe.market is excellent. Check prices there before you decide whether to ducat or sell.

tldr divide ducat value by 7.5 and compare to market; if close, ducat

When you open a relic, the font color of the prime part tells you something of its rarity. Bronze-silver-gold denote common, uncommon and rare, respectively. Indistinguishable in this color scheme are tiers between common-uncommon and uncommon-rare. Ducat values will be either 15 or 25 for bronze, 45 for uncommon, and either 65 or 100 for rare.

What people generally refer to as junk (eg buying or selling 5:10= 2p each) are the pieces worth 15 ducats, but all rarities have junk. This ratio of 7.5 ducats to 1p is useful, fair and reasonably accurate: 25d= 3p, 45d= 6p, 65p= 8p, 100d= 13p.. but it can definitely be improved upon.

Still, if the ducat value of a piece divided by 7.5 lands near the market price, it's a good candidate for converting to ducats.

Be aware of the prime vault and its contents. Vaulted commons can well remain junk (still check), but uncommons, rares and especially whole sets can appreciate quite nicely. You'll do well to hold onto anything that is soon to enter the vault. Maybe also hang onto extras of particularly meta weapons.