what is the best way to check it?
Here's how I check amp ICs:
1) Visual inspection. Is it burned? Severly discolored? Also look at the PCB under where the amp is mounted for severe discoloration. Also look at the solder joints and make sure the heat sink is present and well connected (thermally) to the tab of the amp IC.
2) Look at the amp IC and see what it's part number is. Google it and find a datasheet. Find the pinout on the datasheet. OR... pull up the schematics to see the amp's pinout. On the pinout, identify the power and ground pins. Use DMM to verify power at the amp.
3) Use oscilloscope to probe the amp's output and input pins. You'll need to operate the game in such a manner so as to cause it to generate sound (i.e. start a game) before you can expect to see any signals. If the output is present, you've got an issue with an output coupling cap, or with traces, or with cabinet wiring. If the output is missing, but the input is present... the amp is bad. If there is no input signal, the amp may be good, because there's a problem further upstream.
If you don't have an oscilloscope, a pair of amplified ead phones, or audio an audio probe (homebuilt or otherwise) may suffice. I use a scope, 'cause I have it.