I have some solar panels, that I don't have any information about it. How can i measure how much current it is able to deliver? With my multimeter under the sunlight? Can I test it with a light bulb if there is no sunlight available at the moment?
As Olin said, Isc (short circuit current) and Voc (Open circuit voltage ) are a good guide when multiplied by an adjustment factor.
What sort and size of solar panel is it?
A photo would help.
For most cells Power_max will be more like 70% of Isc - the more efficient the cell the higher % of Isc that Pmax occurs at.
As Olin says, Vmpp (Volts maximum power point) will probably be at 80-85% of Voc.
So Max Power ~~~~= Isc x 0.7 x Voc x 0.8+ ~= 0.6 x Voc.Isc
You can test with a lightbulb BUT the results will be indeterminate.
"1 sun": = 1000 Watt/metre^2 = notional sunlight at midday with no cloud. Real level varies with location and can be 1.2+ x as high in some places.
A strong halogen lamp (100 Watt +) at a "this is getting too hot for my hand" distance will approach about "1 sun" BUT the spectrum is wrong with far too much IR and not enough visible light.
Almost touching a self ballasted 150 Watt mercury lamp with a small panel so whole panel is near light and moved around to best output point is perhaps 50% of one sun. Perhaps.
Placing the PV panel against a LARGE CFL with flat bars or against a fluorescent lamp tube may give 10%-20% of "one sun". .
To get maximum output you need to test them outside at near midday exposed to cloud free sun and with panel pointing directly at the sun.
Note that output will fall somewhat as the panel heats.
Expected output are from as high as about 140 Watts per square metre of active material with the best silicon crystalline panels, down to about 60 Watts per square metre for older amorphous silicon. There are other types of PV material available but these are liable to be less common.
Efficincies - guideline only. Expect real world to vary somewhat. From page
Amorphous silicon - Glass front with dark to almost black cells 0 can have cells visible as here or little or no indication of individual cells.
Glass, aluminum frame - larger than smaller = crystalline silicon.
Monocrytalline / Polycrystalline / Amorphous
Plasticy look top surface , slightly curved to rounded edges all round, blue todark blue cells with individual cells discernible = Epoxy finish crystalline silicon.
As above but flatter finish with sharper edges - PET crystalline silicon.
Put the panel in full sunlight, then measure the open circuit voltage and the short circuit current. The two together should give you a pretty good idea what the panel is capable of. As a rough guide, figure it can do 80% of the short circuit current at 80% of the open circuit voltage.
For a better figure, test with different resistances around that point. Eventually you can create a plot of voltage and current pairs in full sunlight and find the best operating point for that insolation.
A lightbulb is nowhere near the same as sunlight. Unless you have a special calibrated lamp, which it sounds like you don't, readings with ordinary lightbulbs won't tell you anything useful.