First off remove the oil (710) cap and inside it will have the month and year of the engine.

The following was copyed from Wikipedia:

EJ20
2.0 Litre SOHC or DOHC

[edit] EJ20T
This is not actually a valid code from Subaru, but is mostly used by enthusiasts (and can be seen in such context on webforums such as NASIOC)& also mechanics to describe the entire line of 2.0 litre turbocharged engines that have been available over time. When referring to the EJ20T, one is speaking of one of the following:

[edit] EJ20G
EJ20G fall in to 2 categories -early engines 1989-9/1996 -late wagon and automatic sedans from 9/1996 and later

to identify an early EJ20G

Coil on plug
Divorced idle air controller
Upturned inlet in turbo
Air to water or slated intercooler
Late model EJ20Gs are the same general design as the EJ20K

Usage:

Legacy RS 89-93
Legacy RS-RA 89-93
Legacy GT 89-93
Impreza WRX 92~96
Impreza WRX Wagon 92~98
Impreza WRX STi 94~96

[edit] EJ20K
to identify an EJ20K

Wasted spark coil pack on center of manifold
Inlet under manifold
Divorced idle air controller
Usage:

Impreza WRX & STi 9/96~9/98

[edit] EJ205
This engine series is used for non-Japanese marketed WRX models in the world market as of 1999. The Japanese WRX models use the EJ207 from 1999~2001, except the 5-door wagon which also uses the EJ205. After 2001, all WRXs use the EJ205. to identify an EJ205:

Coil on plug
idle air integrated into throttle body
Usage: Impreza WRX 99~current

[edit] EJ207
to identify an 9/99-9/2000 EJ207

Wasted spark coil pack off center of manifold
Inlet under manifold
Red manifold
Usage:

Impreza WRX STi 9/98~00 (JDM, specifically Homologation models for World_Rally_Championship)

2.0L Quad Cam Twin-turbo from JDM 1996 Subaru Legacy GT
[edit] EJ20TT
This can refer to one of the 2.0 Litre DOHC Sequential Twin Turbo and intercooled engines (EJ20H/EJ20R), used in specialty Legacy's. Yet like EJ20T, it is not actually a valid code used by Subaru themselves.

Usage:

Legacy GT & B4 (JDM)

[edit] EJ20H
Usage:

Legacy BD/BG5 JDM GT's (183 kW manual & auto) and GTB's (190 kW auto)

[edit] EJ20R
Usage:

Legacy BD/BG5 JDM RSB and GTB's (205 kW Manual)
B4 Legacy 280bhp