How to charge these batteries
On the packs there is written 6V (nominal) and it is clear that they are made of 5 cells, thus it is 6V/5 cells = 1.2V per cell.

This leads to the obvious conclusion that they are either NiCd or NiMH.

It is not possible to tell the two chemistries apart by only determining the cell voltage, since it is essentially the same for both NiCd and NiMH.

You could try to peel off the heat shrink enveloping one of the packs (that blue plastic material): if you are lucky the internal cells could bear some marking that could tell you whether they are NiCd or NiMH.

The charging process is not radically different between the two chemistries (they should be charged with a constant current charger), but if you want to fast-charge them the end-of-charge detection method is radically different.

Trickle charge strategy is not that different, though. Note: trickle charge (or slow charge) is when you keep a constant current flowing through the cells without damaging them. NiCd and NiMH allow trickle charging, although with different levels of current.
NEVER trickle charge a LiION or LiPo battery, instead!

Slow charge is convenient when you are not in a hurry because you don't need to stop charging the cells when they are fully charged, so you don't need a sophisticated end-of-charge circuit to turn off the charger when it's done its work.

Keep in mind that NiCd are intrinsically more tolerant to abuse than NiMH, so if you trickle charge the packs at a C/40 rate (C=1500mA for your packs), i.e. you place them under constant current charge at about 38mA, that will be safe for both chemistries. NiCd will tolerate a trickle charge of C/10 at least, but if they are NiMH you could damage them.

You can find more details in this TI document: SNVA557 application note. Relevant excerpts (emphasis mine):


  Slow Charge
  
  Slow charge is usually defined as a charging current that can be applied to the battery indefinitely without damaging the cell (this method is sometimes referred to as a trickle charging).
  
  The maximum rate of trickle charging which is safe for a given cell type is dependent on both the battery chemistry and cell construction. When the cell is fully charged, continued charging causes gas to form within the cell. All of the gas formed must be able to recombine internally, or pressure will build up within the cell eventually leading to gas release through opening of the internal vent (which reduces the life of the cell).
  
  This means that the maximum safe trickle charge rate is dependent on battery chemistry, but also on the construction of the internal electrodes. This has been improved in newer cells, allowing higher rates of trickle charging.
  
  The big advantage of slow charging is that (by definition) it is the charge rate that requires no end-of-charge detection circuitry, since it can not damage the battery regardless of how long it is used. This means the charger is simple (and very cheap).
  
  The big disadvantage of slow charge is that it takes a long time to recharge the battery, which is a negative marketing feature for a consumer product.
  
  Slow Charge Rates
  
  NI-CD: most Ni-Cd cells will easily tolerate a sustained charging current of c/10 (1/10 of the cell's A-hr rating) indefinitely with no damage to the cell. At this rate, a typical recharge time would be about 12 hours.
  
  [...]
  
  NI-MH: Ni-MH cells are not as tolerant of sustained charging: the maximum safe trickle charge rate will be specified by the manufacturer, and will probably be somewhere between c/40 and c/10.
  If continuous charging is to be used with Ni-MH (without end-of-charge termination), care must be taken not to exceed the maximum specified trickle charge rate.


If you need fast charging, though, you definitely need an end-of-charge circuit, otherwise you could easily destroy the cells.

Note that it is possible to tell the two chemistries apart, because (read the cited application note) the chemical reaction that happens during charging is endothermic for NiCd (cells get slightly cooler) whereas it is exothermic for NiMH (cells get hotter). But this is tricky to do, especially if you don't want to risk a cell damage and so you use slow charge.

Using slow charge the change in temperature is very little, so you'd need a sensible temperature sensor attached to the cells, and probably a thermostatic environment to avoid that external influences (like air flow) render your measurement meaningless. Maybe a Styrofoam box and a multimeter with a thermocouple probe could do the trick, but I'm just guessing.