You have to triangulate on the answer, but the Units listing and the Resources listing (both under the Info Panel -- top left corner -- normally shows what tech you are currently researching) will help.
Under Resources, the Strategic Resources listing should show how much iron you have and how much is being used. This combines both iron you have improved plus iron from CS allies plus iron from AI trades, minus the Iron you may have traded away to another AI and minus iron you have used. Let's say it says you have 6 iron and 6 used. To check trades (maybe you traded away 1 iron), check the Diplomacy/Deals tab. Assuming no trades, what units, besides the swordsmen, are using the iron?
Go to the Units listing and sort by name. The most common iron-using units are frigates, swordsmen and longswords (and their various unique unit equivalents). If you are unsure of whether a unit requires iron, use the in-game Civilopedia to quickly check whether the unit requires iron.
Under Resources, the Strategic Resources listing should show how much iron you have and how much is being used. This combines both iron you have improved plus iron from CS allies plus iron from AI trades, minus the Iron you may have traded away to another AI and minus iron you have used. Let's say it says you have 6 iron and 6 used. To check trades (maybe you traded away 1 iron), check the Diplomacy/Deals tab. Assuming no trades, what units, besides the swordsmen, are using the iron?
Go to the Units listing and sort by name. The most common iron-using units are frigates, swordsmen and longswords (and their various unique unit equivalents). If you are unsure of whether a unit requires iron, use the in-game Civilopedia to quickly check whether the unit requires iron.