You can sign up for free on dfs-ais.de (site also available in English) and when filing a flight plan they have questionmarks next to each field which show the options.
CAUTION: Some equipment suffixes might have a slightly different definition to the standard ICAO flight plan due to differences in requirements between EASA & ICAO. But it should be a good guide in general.
Excerpt from field 10 (equipment):
S - Standard COM/NAV equipment serviceable (includes VHF RTF, VOR, ILS)
N - NIL (No COM/NAV equipment or unserviceable)
A - GBAS landing system
B - LPV (APV with SBAS)
C - LORAN C
D - DME
E1 - FMC WPR ACARS
E2 - D-FIS ACARS
E3 - PDC ACARS
F - ADF
G - GNSS
H - HF RTF
I - Inertial Navigation
J1 - CPDLC ATN VDL Mode 2
J2 - CPDLC FANS 1/A HFDL
J3 - CPDLC FANS 1/A VDL Mode A
J4 - CPDLC FANS 1/A VDL Mode 2
J5 - CPDLC FANS 1/A SATCOM (INMARSAT)
J6 - CPDLC FANS 1/A SATCOM (MTSAT)
J7 - CPDLC FANS 1/A SATCOM (Iridium)
K - MLS
L - ILS (remove if Standard COM/NAV equipment selected)
M1 - ATC RTF SATCOM (INMARSAT)
M2 - ATC RTF (MTSAT)
M3 - ATC RTF (Iridium)
O - VOR (remove if Standard COM/NAV equipment selected)
R - PBN approved
T - TACAN
U - UHF RTF
V - VHF RTF (remove if Standard COM/NAV equipment selected)
W - RVSM approved
X - MNPS approved
Y - VHF with 8.33 kHz channel spacing capability
Z - Other equipment carried or other capabilities
Field 18:
NAV/:
Specify RNAV equipment (except PBN), if field 10 includes "G" and/or "Z".
RNAVX and/or RNAVINOP shall be included here if applicable.
PBN/:
A1 - RNAV 10 (RNP 10)
B1 - RNAV 5 all permitted sensors
B2 - RNAV 5 GNSS
B3 - RNAV 5 DME/DME
B4 - RNAV 5 VOR/DME
B5 - RNAV 5 INS or IRS
B6 - RNAV 5 LORAN C
C1 - RNAV 2 all permitted sensors
C2 - RNAV 2 GNSS
C3 - RNAV 2 DME/DME
C4 - RNAV 2 DME/DME/IRU
D1 - RNAV 1 all permitted sensors
D2 - RNAV 1 GNSS
D3 - RNAV 1 DME/DME
D4 - RNAV 1 DME/DME/IRU
L1 - RNP 4
O1 - Basic RNP 1 all permitted sensors
O2 - Basic RNP 1 GNSS
O3 - Basic RNP 1 DME/DME
O4 - Basic RNP 1 DME/DME/IRU
S1 - RNP APCH
S2 - RNP APCH with BAR-NAV
T1 - RNP AR APCH with RF (special authorization required)
T2 - RNP AR APCH without RF (special authorization required)